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A Targetron-Recombinase System for Large-Scale Genome
Engineering of Clostridia
Tristan Cerisy,
a
William Rostain,
a
Audam Chhun,
a
Magali Boutard,
a
Marcel Salanoubat,
a
Andrew C. Tolonen
a
a
Génomique Métabolique, Genoscope, Institut François Jacob, CEA, CNRS, Univ Evry, Université Paris-Saclay, Évry, France
ABSTRACT Clostridia are a group of Gram-positive anaerobic bacteria of medical
and industrial importance for which limited genetic methods are available. Here, we
demonstrate an approach to make large genomic deletions and insertions in the
model Clostridium phytofermentans by combining designed group II introns (target-
rons) and Cre recombinase. We apply these methods to delete a 50-gene prophage
island by programming targetrons to position markerless lox66 and lox71 sites,
which mediate deletion of the intervening 39-kb DNA region using Cre recombinase.
Gene expression and growth of the deletion strain showed that the prophage genes
contribute to fitness on nonpreferred carbon sources. We also inserted an inducible
fluorescent reporter gene into a neutral genomic site by recombination-mediated
cassette exchange (RMCE) between genomic and plasmid-based tandem lox sites
bearing heterospecific spacers to prevent intracassette recombination. These ap-
proaches generally enable facile markerless genome engineering in clostridia to
study their genome structure and regulation.
IMPORTANCE Clostridia are anaerobic bacteria with important roles in intestinal and
soil microbiomes. The inability to experimentally modify the genomes of clostridia
has limited their study and application in biotechnology. Here, we developed a
targetron-recombinase system to efficiently make large targeted genomic deletions
and insertions using the model Clostridium phytofermentans. We applied this ap-
proach to reveal the importance of a prophage to host fitness and introduce an in-
ducible reporter by recombination-mediated cassette exchange.
KEYWORDS clostridia, engineering, prophage, recombinase
The clostridia are Gram-positive obligately anaerobic bacteria that include human
pathogens as well as plant-fermenting species critical for healthy functioning of soil
and gut microbiomes. Clostridium (Lachnoclostridium)phytofermentans ISDg (1)isa
model plant-fermenting Clostridium that breaks down plant biomass using numerous
carbohydrate-active enzymes (CAZymes) and ferments the resulting hexose and pen-
tose sugars into ethanol, hydrogen, and acetate (2–4). C. phytofermentans is a member
of the Lachnospiraceae family that is abundant in soil (5), dominates the rumen (6), and
includes human gut commensals that play important roles in nutrition and intestinal
health (7). Because of their ability to directly ferment lignocellulose, plant-fermenting
clostridia have industrial potential for the transformation of plant biomass into value-
added chemicals. Clostridia have long been the focus of study due to their diverse
importance to human health, industry, and the environment (8). However, a lack of
methods for genetic manipulation of clostridia has hindered their application in
biotechnology and our ability to study the structure and function of their genomes.
The development of targetrons based on the Lactococcus lactis Ll.LtrB group II intron
(9) enabled targeted chromosome insertions in C. phytofermentans (10) and other
clostridia (11). Targetrons are designed group II introns that can be customized to insert
into specific DNA sequences by a retrohoming mechanism with efficiencies approach-
Citation Cerisy T, Rostain W, Chhun A, Boutard
M, Salanoubat M, Tolonen AC. 2019. A
targetron-recombinase system for large-scale
genome engineering of clostridia. mSphere
4:e00710-19. https://doi.org/10.1128/mSphere
.00710-19.
Editor Garret Suen, University of Wisconsin-
Madison
Copyright © 2019 Cerisy et al. This is an open-
access article distributed under the terms of
the Creative Commons Attribution 4.0
International license.
Address correspondence to Andrew C.
Tolonen, atolonen@genoscope.cns.fr.
Tristan Cerisy and William Rostain contributed
equally to this work. Author order was mutually
agreed upon by the coauthors.
Large-scale genome engineering in
Clostridia by combining targetrons and Cre
recombinase shows importance of prophage
to host fitness @andrew_tolonen
Received 24 September 2019
Accepted 23 November 2019
Published
RESEARCH ARTICLE
Synthetic Biology
November/December 2019 Volume 4 Issue 6 e00710-19 msphere.asm.org 1
11 December 2019
ing 100% of cells (12), obviating the need for antibiotic resistance markers to select for
their insertion. Domain IV of the intron can be modified to carry cargo DNA such as
single or two tandem lox sites (13). Lox sites are 34-bp elements of two 13-bp
palindromic arms separated by an 8-bp spacer that are recognized by Cre, a recombi-
nase that does not require host-encoded factors (14). Cre has previously been applied
in clostridia to excise an antibiotic resistance gene integrated by homologous
recombination (15). Modified lox sequences expand the utility of the Cre/lox system.
For example, lox66 and lox71 sites each contain arm mutations such that their
recombination results in a lox72 site with two mutant arms that is no longer
recognized by Cre (16). Orthogonal spacer mutants such as lox511 and loxFAS do
not recombine with each other (17), permitting simultaneous noninterfering lox
recombinations for recombination-mediated cassette exchange (RMCE). RMCE is a
method by which a plasmid and genomic cassette can be exchanged by recombi-
nation mediated by a site-specific recombinase such as Cre (18).
Here, we describe a way to make large precise deletions (Fig. 1A to D) and insertions
(Fig. 1E to H) in clostridial genomes using C. phytofermentans as a model. Targetrons
and Cre recombinase have been used together to make genomic insertions in Esche-
richia coli and deletions in E. coli and Staphylococcus aureus (13), showing that these
tools can be used together in Gram-negative and Gram-positive bacteria. We devel-
oped a targetron-recombinase system for clostridia and applied it to excise 50 genes
(cphy2944 to cphy2993) comprising a 39-kb prophage region in the C. phytofermentans
genome, which we chose because mobile elements such as prophages often reduce
genomic stability and their removal increases fitness in other Gram-positive bacteria
FIG 1 Overview of genomic deletion and insertion in C. phytofermentans. (A) pQlox71 is introduced for genomic
insertion of a lox71 (L71) site using the LtrA protein encoded by the targetron. (B) pQlox71 is cured and pQlox66
is introduced for genomic insertion of a lox66 (L66) site. (C) pQlox66 is cured, and pQcre1 is introduced for
Cre-mediated recombination to delete the sequence between the lox66 and lox71 sites. (D) In the resulting strain,
the deletion and lox72 site are confirmed by PCR (arrows show primers). (E) pQadd1 is introduced for genomic
delivery of a lox511/71 (L5-71) and loxFAS/66 (LF-66) cassette into the genome. (F) pQadd1 is cured and pQadd2
is introduced, bearing the desired insertion sequence flanked by lox511/66 (L5-66) and loxFAS/71 (LF-71) sites. (G)
pQcre2 is introduced for Cre-mediated RMCE. (H) The resulting strain has a genomic copy of the insert sequence
flanked by lox511/72 (L5-72) and loxFAS/72 (LF-72) sites in the genome, which is confirmed by PCR (arrows show
primers).
Cerisy et al.
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(19). We also inserted an anaerobic flavin mononucleotide-based fluorescent protein
(20) into the C. phytofermentans genome by RMCE between genomic and plasmid-
based tandem lox cassettes and demonstrated that it acts as a carbon source-inducible
reporter. We discuss how these methods are an important component of an emerging
suite of tools to engineer clostridial genomes.
RESULTS
Plasmids for genomic deletion and insertion. Our strategy to make targeted
deletions (Fig. 1A to D) and insertions (Fig. 1E to H)intheC. phytofermentans genome
is based on sequential introduction and removal of conditionally replicating plasmids
carrying lox sites or Cre recombinase. We constructed plasmids for targetron-mediated
delivery of lox sites by inserting the lox cassettes into the unique MluI site in domain
IV of the intron, a site that supports DNA insertions while retaining targetron activity
(13). Targetrons can be programmed to insert into the genome in either orientation,
but both lox linkers must be in the same relative orientation in the genome for Cre to
delete the intervening region (Fig. 1C). Similarly, RMCE requires two Cre-based recom-
binations between tandem lox sites whose orientation determines that of the inserted
DNA (Fig. 1E to H). We thus constructed lox-containing targetrons in pQint (10) with
either orientation of lox sites: lox71 (pQlox71F and pQlox71R) and lox66 (pQlox66F and
pQlox66R) for genome deletions and a tandem lox511/71-loxFAS/66 cassette (pQadd1F
and pQadd1R) for genome insertion by RMCE (Table 1; see also Fig. S1 in the supple-
mental material). These plasmids all contain the erythromycin resistance gene from
Streptococcus pneumoniae Tn1545 (21) for selection in E. coli and C. phytofermentans
and the pAM

1 origin that replicates stably in C. phytofermentans but can be cured by
serial transfer in liquid medium lacking antibiotics (10).
TABLE 1 Plasmids and strains used in this study
Plasmid or Strain Description Source or reference
Plasmids
pAT19 erm (erythromycin resistance), pAM

1 origin, pUC origin, oriT 21
pQint pAT19 with targetron (Ll.LtrB-deltaORF intron, ltrA) expressed from
P3558 promoter
10
pRAB1 Source of PpagA-cre cassette 22
pRK24 RP4 conjugal genes, Tet
r
, Amp
r
10
pMTL82351 aad9 (spectinomycin resistance), pBP1 origin, colEI origin, oriT CHAIN Biotech
pMTC6 MlsR, Amp
r
,E. coli-Clostridium shuttle vector. PpFbFPm,lac operator,
thl promoter, thl terminator
32
pQlox71F, pQlox71R pQint with lox71 in either the forward (F) or reverse (R) orientation
inserted into MluI site
This study, Addgene 135655, 135656
pQlox66F, pQlox66R pQint with lox66 in either the forward (F) or reverse (R) orientation
inserted into MluI site
This study, Addgene 135657, 135658
pQcre1 pAT19 with PpagA-cre inserted between EcoRI and XbaI sites This study, Addgene 135659
pQcre2 pMTL82351 with PpagA-cre cassette from pQcre1 inserted in EcoRI site This study
pQadd1F, pQadd1R pQint with lox511/71 and loxFAS/66 cassette inserted into MluI site This study, Addgene 135660, 135661
pQadd2 pAT19 with lox511/66-loxFAS/71 cassette inserted between the PstI
and EcoRI sites
This study. Addgene 135662
pQadd2_P3368_FbFP pQadd2 with P3668-FbFP inserted between SpeI and XhoI sites of
the lox cassette
This study
Strains
E. coli 1100-2 mcrA0,endA1, mcrB9999 strain used as a conjugal donor 10
E. coli NEB 5-alpha Competent E. coli cells used for gene cloning New England BioLabs
C. phytofermentans ISDg Reference strain ATCC 700394
C. phytofermentans IntR2944 cphy2944::lox71F.int2944.591s This study
C. phytofermentans IntF2944 cphy2944::lox71R.int2944.526a This study
C. phytofermentans DI-AS cphy2944::lox71R.int2944.591s, cphy2993::lox66F.int2993.177a This study
C. phytofermentans DI-SS cphy2944::lox71F.int2944.526a, cphy2993::lox66F.int2993.177a This study
C. phytofermentans Del-AS DI-AS-derived cphy2944-cphy2993 deletion strain with palindromic scar This study
C. phytofermentans Del-SS DI-SS-derived cphy2944-cphy2993 deletion strain with lox72 site This study
C. phytofermentans Int1575 cphy1575::lox511/71_loxFAS/66.96a This study
C. phytofermentans Int1575-FbFP cphy1575::lox511/72_P3368-FbFP_loxFAS/72.96a This study
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To express Cre recombinase in C. phytofermentans for genomic deletion between
lox71 and lox66 sites, we constructed pQcre1 (Fig. S1; Table 1) in which cre is expressed
from the Bacillus anthracis PpagA promoter, a well-characterized constitutive promoter
that is widely functional in Gram-positive bacteria (22). Initially, we attempted to deliver
a tandem lox cassette and cre on a single plasmid for RMCE but were unable to
construct such a plasmid likely because of interactions between Cre and the lox
cassette, requiring us to cotransform C. phytofermentans with separate plasmids bear-
ing the lox cassette and cre gene. We found that the pBP1 origin from Clostridium
botulinum (23) replicates stably in C. phytofermentans and can be cured using the same
methods as for pAM

1 plasmids and that the aad9 adenyltransferase gene from
Enterococcus faecalis (24) confers spectinomycin resistance in C. phytofermentans. More-
over, we found that plasmids bearing pAM

1 and pBP1 origins can be simultaneously
maintained in the same C. phytofermentans cell, provided they have different antibiotic
resistance markers. Thus, we constructed pQcre2 in which the PpagA-cre cassette from
pQcre1 was inserted into pMTL82351 (Table 1), which carries the pBP1 origin and aad9.
Targetrons position genomic lox sites. Both lox sites must be in the same
orientation for Cre-based recombination to result in deletion of the intervening DNA
(25). To excise the 39-kb prophage region (cphy2944-2993)intheC. phytofermentans
genome (Fig. 2A), pQlox71R was modified to insert a lox71 targetron at 591 bp from the
cphy2944 start codon in the sense direction relative to gene transcription, yielding
pQlox71R.2944. Plasmid pQlox66F.2993 was similarly built to insert a lox66 targetron in
the antisense orientation at 177 bp from the start of cphy2993. Both cphy2944 and
cphy2993 are transcribed on the reverse strand of the genome, such that the lox71 and
lox66 sites inserted by pQlox71R.2944 and pQlox66F.2993 are in the same direction the
genome (Fig. 2A).
Conjugal transfer of lox71 and lox66 targetron plasmids into C. phytofermentans
consistently yielded 10 to 20 transconjugant colonies, of which 80% to 100% contained
the expected targetron insertion, indicating that the efficiency of delivery and integra-
tion of lox-containing targetrons is similar to that in previous targetrons studies
in C. phytofermentans (10,26,27). Following sequential delivery and curing of
FIG 2 Construction of a C. phytofermentans strain with targetron-mediated insertion of a lox71 site in
cphy2944 and a lox66 site in cphy2993. (A) Genome region with the lox-containing targetron insertions
in cphy2944 and cphy2993. Positions of primers used in panels B and E are shown. (B) PCR confirmation
of lox insertions into cphy2944 (primers 2944_1/2) and cphy2993 (primers 2993_1/2) in 3 DI-AS isolates
(DI1 to DI3). DNA chromatograms from DI1 of the lox71 site in cphy2944 (C) and the lox66 site in cphy2993
(D) with the 8-bp central spacer outlined and arm mutations relative to loxP shown in red. (E) Inverse PCR
(primers int_1/2) shows the 3 DI-AS isolates (DI1 to DI3) contain only the 2 expected genomic targetron
insertions. The 3.5-kb band corresponds to the targetron insertion in cphy2944 and the 1.3-kb band to
the insertion in cphy2993.
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pQlox71R.2944 and pQlox66F.2993, PCR amplification of cphy2944 and cphy2993
showed targetron insertions in both genes (Fig. 2B), and sequencing of these PCR
products established the presence of the expected genomic lox71 in cphy2944
(Fig. 2C) and lox66 in cphy2993 (Fig. 2D). We refer to this double insertion strain as
DI-AS (Table 1). Off-site targetron insertions are common (27), which would result
in unexpected recombinations. Thus, we applied an inverse PCR assay (27) using
primers that anneal to the targetron in order to quantify the number and sequence
of genomic targetron insertions. This assay confirmed that DI-AS contains both
expected genomic targetrons without any additional off-site insertions (Fig. 2E).
Cre-mediated prophage deletion. We introduced Cre recombinase into C. phyto-
fermentans strain DI-AS by conjugal transfer of pQcre1 and diagnosed the Cre-mediated
deletion using 3 PCR amplifications: amplicons spanning each genomic lox site and
an amplicon spanning the genomic region between the two lox sites. Cre-mediated
recombination was highly efficient in C. phytofermentans such that when DI-AS was
transformed with pQcre1, all colonies yielded a product using primers 2944_1/2993_2
spanning the region between lox71 and lox66 and no colonies yielded PCR products
using primers flanking each of the individual lox sites (2944_1/2 and 2993_1/2) (Fig. 3A).
FIG 3 Cre-mediated genomic deletion in C. phytofermentans. (A) PCR of the cphy2944 gene (primers
2944_1/2), the cphy2993 gene (primers 2993_1/2), and the 38.9-kb genomic region (primers 2944_1/
2993_2) before pQcre1 transformation (strains DI-SS and DI-AS) and after pQcre1 transformation (strains
Del-SS and Del-AS). (B) DNA chromatogram of the intron fragment in strain Del-AS with palindromic
positions shown in red. (C) DNA chromatogram of the lox72 site in Del-SS with 8-bp central spacer
outlined and arm mutations relative to loxP in red. Model of how Cre-mediated deletion of the genomic
region between lox71 and lox66 sites results in a bidirectional intron that recombines into a 28-bp intron
fragment lacking a lox72 site in strain Del-AS (D) and a unidirectional intron containing a lox72 site in
strain Del-SS (E).
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We refer to the strain with a Cre-mediated deletion as Del-AS (Table 1). While all 10
Del-AS transconjugants yielded PCR products with primers 2944_1/2993_2, the product
was 400 bp rather than the expected 1.5-kb product (Fig. 3A). The sequence of this PCR
product revealed that all that remained of the expected lox72-containing targetron was
a palindromic fragment corresponding to the first 28 bp of the targetron (Fig. 3B).
Cre-mediated recombination between lox sites in opposing targetrons produces a
targetron fragment consisting of a 500-bp inverted repeat that may be unstable. We
tested if the relative orientation of the genomic targetrons influences the final product
of Cre recombination by generating a second C. phytofermentans strain with both
targetrons in the same orientation. To this end, we modified the lox71-targetron of
pQlox71F.2944 to insert in cphy2944 in the sense orientation with respect to transcrip-
tion at 526 bp from the gene start. As the orientation of both the intron targeting and
lox71 are reversed relative to that in pQlox71R.2944, the orientation of lox71 in the
genome is preserved. We constructed a second double insertion strain, DI-SS (Table 1),
with both genomic targetrons in the same orientation by sequential delivery and curing
pQlox71F.2944 and pQlox66F.2993 using the same procedures as for DI-AS. Following
delivery of pQcre1 to DI-SS, we observed by PCR using primers 2944_1/2993_2 that all
10 transconjugants yielded a 1.5-kb product (Fig. 3A) whose sequence confirmed the
expected recombination resulting in a single targetron containing a lox72 site (Fig. 3C).
We refer to this strain as Del-SS (Table 1). We thus conclude the Cre-mediated deletion
in Del-AS results in an inverted repeat that is excised by homologous recombination to
leave only a short palindromic targetron fragment (Fig. 3D), whereas the intact intron
containing a lox72 site in Del-SS is stable in the genome (Fig. 3E).
Prophage excision reduces growth on nonpreferred carbon sources. Wild-type
and DI-SS strains grow similarly on monosaccharides (glucose and xylose), a disaccha-
ride (cellobiose), and a polysaccharide (galactan) (Fig. 4A to D), indicating that targetron
insertions in cphy2944 (phage cell wall peptidase) and cphy2993 (phage DNA-binding
protein) do not affect growth under these conditions. In contrast, growth of Del-SS was
significantly reduced relative to that of the wild-type (WT) on carbon sources other than
glucose (Fig. 4A to D), suggesting that the prophage contributes to cell fitness. The
organization and sequences of the 50 genes in the C. phytofermentans prophage region
are similar to those in Bacillus subtilis bacteriophage SPP1, which has a linear, double-
stranded 44-kb DNA genome with all genes transcribed in the same direction (28).
While it is unknown if this C. phytofermentans prophage can form lytic particles, it
harbors genes for the major phage functions: bacterial cell wall binding and degrada-
tion, phage tail assembly, phage head assembly, and DNA synthesis/replication (see
Fig. S2). However, the contribution of these genes to cell fitness on nonglucose carbon
sources is not evident from gene annotations.
We compared the mRNA expression in C. phytofermentans WT of the 50 prophage
genes on these four carbon sources as measured by transcriptome sequencing (RNA-
seq) (3), revealing that cphy2976 transcription is upregulated 68-fold on cellobiose,
15-fold on xylose, and 86-fold on galactan relative to that on glucose (Fig. 4E to H). As
such, cphy2976 is among the top 10 most highly expressed genes in the cell on
cellobiose and galactan. The cphy2976 gene is cotranscribed with cphy2975 (29)inan
operon that opposes all other genes in the prophage region (Fig. S2). Neither gene
encodes domains of known function nor has significant similarity to genes in the NCBI
database, and so we cannot postulate their roles in the cell. However, our data indicate
that deletion of these 50 putative prophage genes unexpectedly decreased C. phyto-
fermentans growth on nonpreferred carbon sources, and the extremely high upregu-
lation of cphy2976 suggests that it plays an important role in cell fitness under these
conditions.
Genomic insertion by RMCE. To effectuate targeted genomic insertions in C.
phytofermentans by RMCE, the targetron of pQadd1R (Fig. S1) was customized to insert
tandem lox sites with incompatible linkers (lox511/71 and loxFAS/66) at 96 bp from the
start of cphy1575 (Fig. 5A) to produce pQadd1R.1575. The cphy1575 gene is homolo-
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gous to B. subtilis AprX, a nonessential S8 subtilisin serine protease that degrades
heterologous protein in stationary phase (30, 31). The cphy1575 gene is contained in a
cluster of 6 genes with 50% or greater amino acid similarity (cphy1571 to cphy1576) that
likely have overlapping function, further suggesting that inserting heterologous DNA
into cphy1575 would have minimal impact on the cell. We transformed C. phytofermen-
tans with pQadd1R.1575, confirmed the insertion of the intron into cphy1575 by PCR
(Fig. 5B), and cured the plasmid to yield strain int1575 (Table 1). We confirmed that
int1575 did not contain any additional off-site targetron insertions by inverse PCR,
which yielded only the expected 5.1-kb band corresponding to the insertion in
cphy1575 (Fig. 5C).
pQadd2 contains tandem lox511/66 and loxFAS/71 sites (Fig. S1) to facilitate RMCE
with the complementary cassette integrated in the genome of strain int1575 (Fig. 5A).
The lox sites in pQadd2 are separated by SpeI and XhoI sites into which we cloned a
version of the FbFP oxygen-independent green fluorescent protein from Pseudomonas
putida that has been codon optimized for clostridia (32). The FbFP gene is expressed
from the promoter of cphy3368 (P3368), encoding a GH48 cellobiohydrolase (33) whose
transcription is highly upregulated on cellobiose relative to that on glucose (3). The
resulting pQadd2 plasmid bearing the P3368-FbFP cassette was called pQadd2.P3368-
FbFP. We sequentially transformed strain int1575 with pQadd2.P3368-FbFP and
pQcre2. As these plasmids have different origins of replication and antibiotic resistance,
they can be maintained simultaneously in the C. phytofermentans cell (Fig. 5D).
Following transformation of strain 1575 with pQadd2.P3368-FbFP and pQcre2, PCR
amplification of the cphy1575 genomic region (Fig. 5E) of 10 transconjugants showed
FIG 4 Growth of C. phytofermentans WT (), DI-SS (Œ), and Del-SS () strains on glucose (A), cellobiose (B), xylose (C), and galactan (D). Data points are means
from 4 cultures; shaded areas show ⫾standard deviations (SDs). mRNA expression measured by RNA-seq of cphy2944-cphy2993 in C. phytofermentans WT on
glucose (E), cellobiose (F), xylose (G), and galactan (H). Bars show mean log
2
(RPKM) ⫾SD from duplicate cultures; stars show genes differentially expressed on
other carbon sources relative to glucose. RNA-seq measurements and differential expression statistics are based on a previous study (3).
Clostridia Targetron-Recombinase Engineering
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that they all contained the intended insertion of the P3368-FbFP cassette (Fig. 5F). The
sequence of this PCR product confirmed the insertion of the P3368-FbFP cassette
flanked by lox511/72 and loxFAS/72 sites. As such, Cre-based recombination for RMCE
is highly efficient in C. phytofermentans, similarly to genome deletion using Cre. We
cured the plasmids, yielding strain int1575_FbFP (Table 1), and compared its fluores-
cence to that of the WT. The fluorescence of WT and int1575_FbFP cells was similar
when grown on glucose, whereas int1575_FbFP showed elevated fluorescence relative
to that of the WT on cellobiose (Fig. 5G). These results show that FbFP functions as a
fluorescent reporter in C. phytofermentans and that P3368 can be used to regulate
transcription of heterologous genes in a cellobiose-inducible manner.
DISCUSSION
This study describes a strategy to make genomic deletions and insertions by
leveraging two well-characterized genetic tools that are efficient and independent of
host factors. Targetrons delivered both single and tandem lox sites into the C. phyto-
fermentans genome in more than 80% of transconjugants, showing that splicing lox
sites into the targetron does not significantly reduce its efficiency. Further, Cre effec-
tuated recombination between lox sites for genomic deletion and RMCE-mediated
insertion in 100% of colonies. Targetrons and Cre recombinase have already been used
separately in other clostridial species, supporting that this approach can be generally
applied in clostridia. Unexpectedly, we found that if the two genomic targetrons are in
opposite orientations, the lox72 site and most of the targetron sequence are excised to
leave only a short scar following Cre-mediated recombination. While this secondary
recombination can be prevented by controlling the orientation of the lox sites or
potentially by introducing two nonhomologous targetrons based on Ll.LtrB and EclV, it
is potentially preferable to reduce the size of the scar relative to a lox-containing
targetron.
Engineering of clostridia with streamlined genomes will enhance our ability to
optimize the production of target molecules and understand the contributions of
nonessential genes to cell fitness. Deletion of one or a few genes by double-crossover
homologous recombination-based integration of an antibiotic resistance marker has
been achieved in certain clostridial species (34–36). Moreover, CRISPR is proving to be
an efficient way to select for clostridia with markerless, homologous recombination-
mediated gene deletions by killing nonrecombinant cells using Cas nucleases (37–39).
CRISPR methods can select for small genomic changes because Cas nucleases can be
FIG 5 Genomic insertion in C. phytofermentans by RMCE. (A) Diagram of the cphy1575 region in strain int1575 including positions of
primers to confirm the targetron insertion in cphy1575 (1575_1/2) and the number of genomic targetron insertions (int_1/2). (B) PCR
of cphy1575 in WT and int1575 (primers 1575_1/2) strains shows insertion of the targetron containing the lox511/71_loxFAS/66
cassette in int1575. (C) Inverse PCR of int1575 genomic DNA (primers int_1/2) shows int1575 contains a single targetron insertion. The
5.1-kb band corresponds to the expected targetron insertion in cphy1575. (D) Plasmids pQadd2 and pQcre2 can be simultaneously
maintained in strain int1575. PCR compares int1575 before plasmid transfer (⫺for pQAdd1 and pQcre2) and after transfer of both
plasmids (⫹for pQadd1 and pQcre2) using primers to amplify the plasmid origins of replication. (E) Diagram of cphy1575 in
int1575_FbFP after RMCE to insert the P3368-FbFP cassette. (F) PCR of cphy1575 in int1575 (⫺) and int1575_FbFP (⫹) shows insertion
of the P3368-FbFP cassette in int1575_FbFP. (G) Fluorescence (448/20-nm excitation, 495/20-nm emission) of wild-type and
int1575_FbFP strains shows cellobiose-inducible FbFP expression. Bars show mean fluorescence normalized to cell density (OD
600
)
from triplicate cultures ⫾SDs.
Cerisy et al.
November/December 2019 Volume 4 Issue 6 e00710-19 msphere.asm.org 8
programmed to cleave any genomic site containing a PAM sequence (40). However,
CRISPR methods are still limited by the probability of initially introducing the desired
mutation by homologous recombination, because CRISPR-mediated DNA cleavage only
modestly induces recombination in bacteria (40) and most bacterial taxa lack the
proteins for nonhomologous end joining (41). As the efficiency of homologous recom-
bination decreases exponentially with distance between recombination sites (42),
CRISPR methods are generally confined to making genomic changes of a few kilobases
or less. Furthermore, the double-stranded DNA cleavage activities of Cas nucleases are
toxic, making them difficult to introduce into clostridial species with low transformation
efficiencies (41). In contrast, targetrons and Cre-lox are independent of homologous
recombination, do not cause double-stranded DNA breaks, and are efficient such that
large changes can be easily made without selection.
Insertion of heterologous genes and pathways into clostridia is fundamental to
being able to endow them with novel phenotypic and metabolic properties. The C.
phytofermentans int1575 strain containing a genomic lox511/71-loxFAS/66 cassette is a
chassis that can be generally applied for facile RMCE-mediated integration of genes and
pathways of interest. Genomic insertion by RMCE does not require selectable markers,
and the length of the inserted DNA is only limited by that which can be cloned into
pQadd2. A previous method for genomic insertion in clostridia termed allele-coupled
exchange (ACE) links expression of a selectable marker gene to the formation of a
double-crossover recombinant chromosome (43). ACE is advantageous relative to
RMCE in permitting serial accumulation of larger insertion fragments but disadvanta-
geous in requiring a uracil auxotrophy or genomic insertion of antibiotic resistance
genes. In conclusion, the targetron-recombinase method described here expands our
capabilities to engineer clostridia by enabling large markerless genomic deletions and
insertions. This approach complements other recent technologies such as CRISPR and
ACE to make a suite of tools to understand the biology of clostridia and exploit their
usefulness in industry and medicine.
MATERIALS AND METHODS
Cell cultivation and conjugation. C. phytofermentans ISDg (ATCC 700394) was cultured anaerobi-
cally at 30°C in GS2 medium (44) containing 3 g liter
⫺1
of either glucose (G5767; Sigma), xylose (X3877;
Sigma), cellobiose (C7252; Sigma), or galactan (P-GALLU; Megazyme). Growth on different carbon sources
was measured in 400-
l cultures in 100-well microtiter plates (9502550; Bioscreen) that were sealed in
the anaerobic chamber (2% H
2
, 98% N
2
) as previously described (45). Cell densities were measured using
a Thermo Scientific Bioscreen C as the optical density at 600 nm (OD
600
) with 30 s of shaking before each
reading.
Plasmids were introduced into C. phytofermentans by conjugal transfer from E. coli strain 1100-2
(pRK24) (10), and plasmids were maintained using erythromycin (200
gml
⫺1
in liquid medium, 40
g
ml
⫺1
in solid medium) or spectinomycin (600
gml
⫺1
in liquid medium and solid medium). Following
conjugation, ten transconjugant colonies were picked, and the presence of the plasmid was confirmed
by PCR using primers pAMB1_1/2 for pAM

1 origin plasmids and primers pBP1_1/2 for pBP1 origin
plasmids. Plasmids were cured by five successive transfers at 1:100 dilution in 5 ml liquid medium lacking
antibiotics. Plasmid loss was confirmed based on antibiotic sensitivity and lack of a PCR product. The
expected intron insertions in the C. phytofermentans genome were confirmed by PCR and sequencing
using primers flanking the genomic insertion site (primers are listed in Table S1 in the supplemental
material).
Plasmid construction. Enzymes were purchased from New England BioLabs, and cloning was
performed using NEB 5-alpha Competent E. coli cells (C2987I; NEB). The lox66 and lox71 sites were cloned
by annealing complementary 5=phosphorylated oligonucleotides (L71_1/2 and L66_1/2) and cloning
them into the unique MluI site in domain IV of the Ll.LtrB-ΔORF intron of pQint (10). To enable
positioning of lox sites in either orientation in the genome relative to the intron insertion, lox66 and lox71
sites were cloned into pQint in both orientations yielding 4 plasmids: pQlox66F, pQlox66R, pQlox71F, and
pQlox71R. Using crossover PCR, the pQlox71F intron was targeted to insert in the antisense orientation
at 526 bp from the cphy2944 start codon (pQlox71F.2944), the pQlox71R intron was targeted to insert at
591 bp from the cphy2944 start codon (pQlox71R.2944), and the pQintL66F intron was targeted to insert
in the antisense orientation at 177 bp from the start of cphy2993 (pQlox66F.2993). pQcre1 was con-
structed by PCR amplifying the PpagA-cre cassette from pRAB1 (22) using primers PpagA-Cre_1/2 and
inserting it between the EcoRI and XbaI sites of pAT19 (21). PCR and sequencing using primers
OK_PpagA-Cre_1/2 confirmed the presence of the PpagA-cre cassette in pQcre1.
pQadd1F/R were constructed by annealing complementary 5=phosphorylated oligonucleotides
2ML4_1/2 encoding a double lox cassette (lox511/71 and loxFAS/66) and cloning it into the unique MluI
site of pQint. The pQadd1R intron was subsequently targeted to insert into cphy1575 in the antisense
Clostridia Targetron-Recombinase Engineering
November/December 2019 Volume 4 Issue 6 e00710-19 msphere.asm.org 9
orientation at 96 bp from the start codon to make plasmid pQadd1R.1575. pQadd2 was constructed by
annealing and PCR extending oligonucleotides 2ML9_1/2 encoding a double lox cassette (lox511/66 plus
loxFAS/71), and cloning the resultant product between the unique PstI and EcoRI sites of pAT19. The
P3368-FbFP cassette was inserted into pQadd2 by PCR amplifying the cphy3368 promoter region from C.
phytofermentans genomic DNA using primers P3368_1/2 and cloning the product into the SpeI and XhoI
sites between the lox511/66 and loxFAS/71 sites of pQadd2. The PpFbFPm gene was PCR amplified from
pMTC6 (32) using primers P3368_FbFP_1/2 and cloned between the XbaI and XhoI sites directly
downstream of P3368 to make pQadd2.P3368-FbFP. To enable cotransformation of pQadd2.P3368-FbFP
and a cre plasmid into the same cell, pQcre2 was built by PCR amplifying the PpagA-cre cassette from
pQcre1 using primers Cre2_1/2 and subcloning it into the unique EcoRI site of pMTL82351 (CHAIN
Biotech). The sequences of all plasmids were confirmed by Sanger sequencing.
Genomic insertion of lox sites. The C. phytofermentans strains carrying lox71and lox66 sites for
prophage deletion were made by sequential conjugal delivery and curing of plasmids pQlox71R.2944
and pQlox66F.2993 (strain DI-AS) or plasmids pQlox71F.2944 and pQlox66F.2993 (strain DI-SS). Following
delivery of each plasmid, we confirmed the intended targetron insertion in 10 transconjugants by PCR
using primers flanking the insertion site, cured the plasmid from 3 transconjugants, and demonstrated
plasmid loss by PCR and restoration of erythromycin sensitivity. Following delivery and curing of
plasmids for both lox sites, PCR and sequencing confirmed the presence and orientation of both the
lox71 site at 591 bp from the cphy2944 start in strain DI-AS and at 526 bp from the cphy2944 start in strain
DI-SS (primers OK_2944_1/2). Similarly, the location and orientation of the lox66 site at 177 bp from the
cphy2993 start were confirmed in both strains (OK_2993_1/2). The C. phytofermentans int1575 strain was
made by conjugal delivery and curing of pQadd1R.1575. The presence and antisense orientation of the
lox511/71_loxFAS/66 cassette at 96 bp from the start of cphy1575 in the int1575 genome was confirmed
in 8 of 10 transconjugants by PCR and sequencing (OK_1575_1/2).
The absence of additional, off-site genomic targetron insertions in strains DI-SS, DI-AS, and int1575
was confirmed by inverse PCR (27). To this end, genomic DNA was extracted and purified from 5 ml of
log-phase culture using the GenElute Bacterial Genomic DNA kit (NA2110; Sigma). DI-SS and DI-AS
genomic DNA (1
g) was digested with HindIII; int1575 genomic DNA was digested with EcoRI. Digested
DNA was purified with QIAquick PCR purification kit (28104; Qiagen) and ligated with T4 DNA ligase
(M0202S; NEB). Intron insertion sites were PCR amplified by Q5 polymerase (M0491; NEB) using primers
int_1/2 with a 7-min extension time. Targetron insertion sites were identified by gel extraction and
sequencing of the PCR products.
Genomic deletion and insertion using Cre recombinase. To effectuate Cre-mediated genomic
deletions, pQcre1 was conjugated into C. phytofermentans strains DI-SS and DI-AS, and its presence in 10
transconjugants was confirmed by PCR (primers pAMB1_1/2). Liquid cultures of the ten transconjugants
were grown in liquid medium containing erythromycin, and deletion of the 38.9-kb region between the
lox71 and lox66 sites was diagnosed using 3 PCRs: two reactions to confirm the absence of products
using primers 2944_1/2 and 2993_1/2 and a reaction to amplify a product spanning the deleted region
using primers 2944_1/2993_2. pQcre1 was cured from Del-SS and Del-AS as described above.
To make a Cre-mediated genomic insertion by RMCE, pQadd2.P3368-FbFP and pQcre2 were sequen-
tially conjugated into strain int1575, and the simultaneous presence of both plasmids in 10 transcon-
jugant colonies was confirmed by PCR using primers pBP1_1/2 for pQcre2 and pAMB1_1/2 for
pQadd2.P3368-FbFP. After 1 transfer in medium containing erythromycin and spectinomycin, PCR using
primers OK_1575_1/2 diagnosed insertion of the P3368-FbFP cassette into the int1575 genome, yielding
strain int1575_FbFP.
Measurement of FbFP fluorescence. Cellular fluorescence was compared between triplicate cul-
tures of C. phytofermentans WT and strain int1575_FbFP. Cultures were grown to log phase (OD
600
of 0.6)
in glucose or cellobiose medium. One milliliter of culture was centrifuged, and the cell pellet was washed
once in phosphate-buffered saline PBS and resuspended in 500
l PBS. The OD
600
of each culture was
confirmed, and the cellular fluorescence was measured in black 96-well Costar plates using a Safas Xenius
XMA (excitation, 448/20 nm; emission, 495/20 nm; photomultiplier, 850 V). Background fluorescence of
PBS was subtracted from measurements, and the fluorescence measurements were normalized to culture
density (OD
600
).
Data availability. Plasmids from this study have been submitted to the Addgene Plasmid Repository
and are available using the accession numbers listed in Table 1.
SUPPLEMENTAL MATERIAL
Supplemental material for this article may be found at https://doi.org/10.1128/
mSphere.00710-19.
FIG S1, EPS file, 0.2 MB.
FIG S2, EPS file, 0.1 MB.
TABLE S1, XLSX file, 0.1 MB.
ACKNOWLEDGMENTS
We thank Ralph Bertram and Christopher F. Schuster from the University of Tuebin-
gen, Germany, for providing pRAB1, Corinne Cruaud for DNA sequencing, and Peter
Enyeart for helpful discussions.
Cerisy et al.
November/December 2019 Volume 4 Issue 6 e00710-19 msphere.asm.org 10
This work was funded by the Genoscope-CEA and the Agence Nationale de la
Recherche Grant ANR-16-CE05-0020.
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