Phototaxis and impaired motility in adenylyl cyclase and cyclase receptor protein mutants of Synechocystis sp. strain PCC 6803.

Abstract

We have carefully characterized and reexamined the motility and phototactic responses of Synechocystis sp. adenylyl cyclase (Cya1) and catabolite activator protein (SYCRP1) mutants to different light regimens, glucose, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, and cyclic AMP. We find that contrary to earlier reports, cya1 and sycrp1 mutants are motile and phototactic but are impaired in one particular phase of phototaxis in comparison with wild-type Synechocystis sp.

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JOURNAL OF BACTERIOLOGY, Oct. 2006, p. 7306–7310
0021-9193/06/$08.00?0 doi:10.1128/JB.00573-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Vol. 188, No. 20
NOTES
Phototaxis and Impaired Motility in Adenylyl Cyclase and Cyclase
Receptor Protein Mutants of Synechocystis sp. Strain PCC 6803
Devaki Bhaya,1* Kenlee Nakasugi,2Fariba Fazeli,1and Matthew S. Burriesci1
The Carnegie Institution, 260 Panama Street, Stanford, California 94305,1and School of Biotechnology and
Biomolecular Sciences, The University of New South Wales, Sydney, NSW 2052, Australia2
Received 23 April 2006/Accepted 31 July 2006
We have carefully characterized and reexamined the motility and phototactic responses of Synechocystis sp.
adenylyl cyclase (Cya1) and catabolite activator protein (SYCRP1) mutants to different light regimens, glucose,
3-(3,4-dichlorophenyl)-1,1-dimethylurea, and cyclic AMP. We find that contrary to earlier reports, cya1 and
sycrp1 mutants are motile and phototactic but are impaired in one particular phase of phototaxis in compar-
ison with wild-type Synechocystis sp.
In a screen designed to identify transposon-tagged motility
mutants in Synechocystis sp. strain PCC 6803 (hereafter Syn-
echocystis sp.), we identified a transposon insertion in the gene
encoding adenylyl cyclase (cya1 or slr1991) which caused an
apparent nonmotile phenotype (5). Adenylate cyclase (AC)
synthesizes 3?,5?-cyclic AMP (cAMP) from ATP. cAMP is a
ubiquitous second messenger that participates in a wide variety
of signal transduction systems in bacteria and in eukaryotes (1,
2). cAMP binds to a dimer of the catabolite receptor protein
(CRP; also known as catabolite activator protein) which re-
quires the allosteric effector cAMP in order to bind efficiently
to DNA. In Escherichia coli CRP activates transcription at
more than 100 promoters, by binding to a well-conserved pal-
indromic binding motif (TGTGAN6TCACA). In Synechocystis
sp. inactivation of CRP (sycrp1, sll1371) resulted in an appar-
ently nonmotile phenotype (27, 28).
Six classes of adenylyl cyclases (I to VI) with structurally
unique catalytic domains are found in prokaryotes (1, 8, 18).
Of these only the class III universal class of ACs is found
among both prokaryotes and eukaryotes (21). All cyanobacte-
rial ACs share homology with the catalytic domain of eukary-
otic adenylate cyclases but often have other functional domains
fused at either the C or the N terminus of the protein and are
therefore likely to be multifunctional (17, 18). Cya1 (encoded
by cya1) is the major class III AC in Synechocystis sp.; cya3
(sll1161) encodes an AC-like product but lacks several critical
residues, suggesting that it may not be functional; cya2
(sll0646) encodes a guanylyl cyclase (15, 16). sycrp1 (sll1371)
and sycrp2 (sll1924) encode cAMP receptor-like proteins in
Synechocystis which are homologous to each other, although
Sycrp2 lacks several residues required for cAMP binding (26).
Anabaena cylindrica and Synechocystis sp. exhibit rapid
changes in cAMP levels triggered by different light qualities,
including UV-A and UV-B (7, 12, 13, 18). When Synechocystis
sp. is exposed to white light (100 ?mol photon m?2s?1) after
incubation in the dark for 40 min, it shows a rapid increase in
intracellular cAMP content. cAMP levels increased in re-
sponse to blue light (450 nm) and UV-A light (380 nm), but no
other wavelengths (520, 575, 670, or 720 nm) induced this
response. Furthermore, 63% of the cells were motile under
blue light versus 24% in red light, suggesting that cAMP might
mediate blue light signals, and it has been suggested that a
BLUF domain-containing protein encoded by slr1694 may be
involved in cAMP-mediated blue light signal transduction (23).
Inactivation of this protein abolished positive phototaxis (19),
but cAMP levels in the mutants were the same as in the wild
type (WT) under specific light regimens (13).
To characterize phototaxis and motility in Synechocystis sp.,
WT cells were spotted on motility plates and grown in direc-
tional white light (5). Based on the motility behavior of indi-
vidual cells, we identified three phases of motility. In phase 1,
we observed single cells or cells in small groups on the agarose
surface which exhibit limited motility and phototaxis (Fig. 1A
and D). In the next observable phase (phase 2), which usually
occurred between 16 and 48 h after the cells were spotted, cells
had aggregated into larger groups which clearly exhibited pho-
totaxis (Fig. 1B and E). Cells nearest the light source had
moved and accumulated at the front edge of the spot, forming
a crescent shape. Cells at the back or the center of the spot also
exhibited phototaxis. In phase 3, fingerlike projections or a
moving front of cells extended from the front edge of the spot
toward the light source, and within these projections cells
moved rapidly (Fig. 1C). Within 2 to 4 days after spotting, the
characteristic fingerlike projections extended ?4 to 10 mm,
depending on light intensities and the degree of surface wet-
ness of the plate. The rates of movement of cells and the gross
morphology of the moving front of cells appeared to be a
function of cell density and cell doubling times, surface wet-
ness, and light quality.
Spectinomycin-resistant strains with targeted mutations of
cya1 and sycrp1 were created using a standard PCR-based
inactivation strategy. To inactivate cya1 (slr1991) and sycrp1
* Corresponding author. Mailing address: Carnegie Institution, 260
Panama Street, Stanford, CA 94305. Phone: (650) 325 1521, ext. 282.
Fax: (650) 325-6857. E-mail: dbhaya@stanford.edu.
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(sll1371), genes were amplified from genomic DNA using
primers (forward primer 5? ATGGGCACTAGTCCCCAA 3?
and reverse primer 5? TCAAGGAAATTAGATCTT 3? for
sll1371 and forward primer 5? GTGGATAAGCCTGCCCTA
3? and reverse primer 5? TTAGGGCCCTTCCGAGGC 3? for
slr1991). PCR products were purified and ligated into pGEM
T Easy vector (Promega). The spectinomycin cassette (di-
gested with SmaI) from plasmid HP?45 was ligated into the
unique SmaI site in sll1371. The resulting plasmid was used
for the transformation of Synechocystis sp. The plasmid con-
taining the 1-kb slr1991 insert was digested with SalI and
blunted, and the spectinomycin cassette (digested with SmaI)
was ligated into the site. Colony PCR was used to assess
whether complete segregation had been achieved, and these
transformants were checked for motility (4).
The cya1 and sycrp1 mutants behaved like WT cells in phase
1 and phase 2 of motility. However, neither the cya1 nor the
sycrp1 mutants made the typical fingerlike projections charac-
teristic of phase 3 (Fig. 2A). Thus, in the cya1 and sycrp1
mutants the cells moved and accumulated at the front of the
spot, making a crescent shape, but movement was retarded
beyond this step. The cya1 mutant cells were evenly distributed
at day 2 (Fig. 2C) but had migrated to the front of the spot by
day 4 (Fig. 2D). Even after several days, the cya1 and sycrp1
mutant cells did not form fingers, suggesting that they were
phototactic but defective in phase 3 motility. As cells continued
to grow and divide, they became confluent, giving the appear-
ance of a nonmotile colony (i.e., they do not make character-
istic fingers associated with WT cells), which explains why
these mutants were initially characterized as being “nonmo-
tile” (5, 22).
In the presence of exogenously added 0.1 mM cAMP, cya1
mutant cells proceeded to phase 3 of motility and regained
wild-type levels of motility and phototaxis; however, the sycrp1
mutant did not regain phase 3 motility (Fig. 2B). Even higher
concentrations of cAMP did not rescue the sycrp1 mutant (data
not shown), suggesting that in the absence of the receptor
protein, the exogenous addition of cAMP has no effect on
FIG. 1. Phases of phototaxis in Synechocystis sp. cells. Cells were spotted on motility plates, placed in unidirectional white light, and photographed. After
overnightgrowth,cellsareeithersingleorinsmallgroups;thefaintgreenedgevisibleatthefrontofthespotiscausedbycellsthathavemovedandaccumulated
at the front edge (A and D); after 2 days (B and E), many more cells are in groups and have migrated to the front of the spot, typical of phase 2; after 4 days
(C), fingerlike projections of motile cells are seen, typical of phase 3. Lower panels: single cells (D) and cell groups (E) from spots in panels A and B are shown
athighermagnification.Motilityplatescontained0.4%agaroseinBG-11and10mMglucose.Thearrowshowsthedirectionofthelight.Spotsare?3to4mm
in diameter. Bar, 10 ?m.
FIG. 2. Effect of cAMP on phototaxis of Synechocystis sp. WT and
cya1 and sycrp1 mutant cells. Upper panels: WT (left), cya1 mutant (mid-
dle), and sycrp1 mutant (right) cells were spotted on 0.4% agarose BG-11
motility (no glucose added) plates in the absence (A) or presence (B) of
0.1 mM cAMP and photographed after 4 days. Lower panels: the front
edge of the spot containing cya1 mutant cells is shown after day 2 (C) and
day4(D).Notethatcellsareevenlydistributedatday2,butafterday4,cells
have clearly accumulated at the front. In the presence of 0.1 mM cAMP (B),
cya1 mutant cells shows the typical fingerlike projections seen in WT cells;
sycrp1 mutant cells do not respond to cAMP. The arrow shows the direction
of the light. Spots are ?3 to 4 mm in diameter. Bar, 10 ?m.
VOL. 188, 2006NOTES7307

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motility. If the action of cAMP is due solely to its role as an
allosteric effector working by binding to Sycrp1, this result is to
be expected. cAMP levels in Synechocystis cells have been
measured between 600 and 1,000 pmol cAMP mg Chl?1when
cells are transferred to blue light from darkness (13, 23). Cel-
lular cAMP content in the cya1 mutant cells was less than 5%
of that in wild-type cells (22).
We checked the effect of glucose on phototaxis, since glu-
cose has a marked effect on cAMP levels in E. coli and it is
known that glucose can support photoheterotrophic growth in
Synechocystis sp. (6, 29, 30). The addition of increasing con-
centrations of glucose to the solid medium strongly affected
motility (Fig. 3). In the absence of glucose, WT cells moved as
a front, while cya1 and sycrp1 mutant cells accumulated at the
front of the spot as described above. With increasing concen-
trations of glucose (ranging from 0.1 mM to 1 mM) there were
progressively longer projections of motile cells representative
of phototaxis. This rapid phototaxis is likely to be caused by a
combination of factors including an increased cell density (be-
cause of a faster cell doubling time in the presence of glucose)
and perhaps a specific effect, such as the enhanced production
of extracellular material, which assists cell motility.
The cya 1 and sycrp1 mutants appeared to have an enhanced
cell density in the presence of increasing glucose concentra-
tions, and at 5 and 10 mM glucose there is a modest increase
in forward motility but the cya1 mutant cells still do not make
the fingerlike projections typically observed in WT cells. Thus,
addition of glucose alone could not compensate for the ab-
sence of cAMP under these conditions. To separate the pos-
sible effects of glucose on growth from a specific effect on
phototaxis, we checked the effect of 3-(3,4-dichlorophenyl)-
1,1-dimethylurea (DCMU) on phototaxis (24). DCMU is a
specific inhibitor of photosynthetic activity, and cell growth is
strongly inhibited in the presence of 10 ?M DCMU. We found
that in the presence of DCMU, cells did not divide and re-
mained as single cells on the plate (Fig. 4A); however, in the
presence of 10 mM glucose and 10 ?M DCMU (Fig. 4D) some
of the typical fingerlike projections indicative of phototaxis did
appear, indicating that glycolysis can provide some of the en-
ergy (ATP) required for cell division and phototaxis. We ob-
served maximal phototaxis in the presence of 10 mM glucose
and white light (Fig. 4C); in the absence of glucose, slower cell
growth and possibly the lack of other factors, such as polysac-
charide production (10), that enhance motility may reduce the
extent to which cells move (Fig. 4B). Maximal rates of motility
were observed when there were both active photosynthesis
and energy provided from the respiration of exogenously
added glucose. Glucose is phosphorylated by a glucose kinase
(sll0593) and enters the oxidative pentose phosphate pathway,
glycolysis, and the tricarboxylic acid cycle to produce ATP,
NADPH, and carbon skeletons for growth. Doubling times of
Synechocystis in the presence of 10 mM glucose and light (i.e.,
photomixotrophic conditions) were 8.2 h compared to 22.2 h
under photautotrophic conditions (9, 20).
We have previously shown that red light is effective even at
a fluence of about 1 ?mol photon m?2s?1for phototaxis,
although rates of phototaxis were enhanced up to 40 ?mol
photon m?2s?1(14). For the photomotility assays, light inten-
sity was set between 6 and 10 ?mol photons m?2s?1using
light-emitting diode light sources (Roithner Lasertechnik, Aus-
tria, or SuperbrightLEDs.com). Light-emitting diodes were white
(bimodal with a sharp peak at 465 nm and a broader peak be-
tween 559 and 587 nm), red (peak at 636 nm), green (528 nm), or
blue (464 nm).
The motilities of cya1 and sycrp1 mutants and WT cells were
FIG. 3. Glucose effect on motility of Synechocystis sp. WT and cya1
and sycrp1 mutant cells. WT (left), cya1 mutant (middle), and sycrp1
mutant (right) cells were spotted on 0.4% agarose motility plates
containing no glucose (A) or 0.1 mM (B), 0.2 mM (C), 1 mM (D), 5
mM (E), or 10 mM (F) glucose. Plates were placed in directional white
light and photographed after 4 days. Note that cya1 mutant cells show
slight formation of fingerlike projections in 10 mM glucose, but sycrp1
mutant cells do not.
FIG. 4. Motility in the presence of glucose and DCMU. Synecho-
cystis sp. WT cells were spotted on 0.4% agarose BG-11 plates (A) or
plates containing 10 ?M DCMU (B), 10 mM glucose (C), or 10 mM
glucose and 10 ?M DCMU (D). Spots were photographed after 4 days.
After 4 days the cells growing in DCMU have not divided and cannot
be easily seen at this magnification. Spot diameter is ?4 mm.
FIG. 5. Motility of Synechocystis sp. WT and cya1 and sycrp1 mu-
tant cells in different light qualities. WT (left), cya1 mutant (middle),
and sycrp1 mutant (right) cells were spotted on 0.4% agarose BG-
11–10 mM glucose motility plates in the absence (top panels) or the
presence (bottom panels) of 0.1 mM cAMP. Plates were placed in
different light sources (white, red, green, or blue) and photographed
after 4 days. Spot diameter is ?4 mm.
7308NOTES J. BACTERIOL.

Page 4

checked under four different light conditions, either in the
presence or in the absence of cAMP (Fig. 5). In white, red, and
green light, WT cells exhibited rapid phototaxis, both in the
absence and in the presence of cAMP. In white, red, and green
light, in the presence of exogenously added cAMP, the motility
of the cya1 mutant was restored (lower panels) In blue light,
cell division and growth were lower than in other light condi-
tions, but if cells were spotted at a high density (or allowed to
grow for long periods on plates), one observed small fingerlike
projections. In blue light, exogenous cAMP did appear to res-
cue the cya1 mutant but the results were not as obvious as in
the other light regimens since the fingerlike projections were
very small. In contrast to the cya1 mutant, the sycrp1 mutant
cells were not rescued by the addition of cAMP. It has been
postulated that the blue light signal for phototaxis is medi-
ated by cAMP, although the regulatory mechanisms have
not yet been identified (23). It has also been proposed that
the phytochrome Cph2 is a component of a signal transduc-
tion event inhibiting the movement of Synechocystis sp.
strain PCC 6803 cells towards blue light (24). Recently, a
flavin adenine dinucleotide-binding BLUF domain protein
(the slr1694 gene product) has also been suggested to play a
role in cAMP-mediated blue light signaling for positive pho-
totaxis (11, 19, 23).
We used a combination of time-lapse video microscopy and
tracking software to analyze the movement of individual WT
and mutant cells. WT and cya1 mutant cells were spotted at a
low density on motility plates and incubated for 24 h with
directional white light. By this time most cells were in phase 2.
We identified and tracked single cells since tracking groups of
cells is challenging. Movements of wild-type and cya1 mutant
single cells in response to unidirectional white light (95 ?mol
photon m?2s?1) were recorded using a Coolsnap Pro Mono-
chrome (Media Cybernetics, Silver Spring, MD) camera at-
tached to a Nikon TE300 inverted microscope. Movies were
acquired in 120 frames for a total of 1,800 s for each movie,
with the time between frames set at 15 s. The movement of
?20 to 30 individual, well-separated cells was tracked with
Metamorph software (Universal Imaging Corporation, Down-
ingtown, PA) using a combination of automated and manual
tracking. Representative tracks of individual cells (WT cells
and cya1 mutants) over a 30-min period are shown in Fig. 6.
Two motion parameters were measured, the shortest linear
distance or displacement (D) from the start point of a time-
lapse recording to the finish and the total distance or path
length (T) traversed by a cell. The ratio of D/T provided a
measure of directed motility or phototaxis. The average total
path length (T) for WT and cya1 mutant cells was computed to
be 30.2 ? 10.6 ?m and 41.3 ? 9.2 ?m, respectively, over the
30-min period. The total displacement (D) was 15.5 ? 8.1 ?m
and 17.0 ? 4.6 ?m, respectively, for WT and cya1 mutant cells.
Thus, the average of the D/T ratios of individual cells was
found to be 0.44 ? 0.08 and 0.44 ? 0.13 for WT and the cya1
mutant, respectively. The mean velocities were found to be
1.00 ? 0.35 ?m/min and 1.38 ? 0.31 ?m/min for WT and cya1
mutant cells, respectively. Based on these parameters (i.e.,
directed motility and velocity) individual cya1 mutant cells are
indistinguishable from WT cells in the early phases of motility.
Once the cells have moved to the front and are packed to-
gether, it is no longer possible to track the movement of single
cells and so we cannot estimate motility rates beyond phase 2.
These results confirm that the cya1 mutant cells are motile and
phototactic but cannot make the fingerlike projections that are
characteristic of the WT cells. sycrp1 mutants also show the
same behavior (data not shown).
In summary, we carried out a detailed characterization of
the phototactic response of Synechocystis sp. and cya1 mutants
to different light regimens, glucose, and cAMP. The results of
our study provide evidence that cya1 and sycrp1 mutants are
motile and phototactic at the individual cell level and the
characteristics of movement (in phases 1 and 2) are very sim-
ilar to those of WT cells. The strong phototaxis that is char-
acteristic of phase 3 (in which WT cells move rapidly forward
in the fingerlike projections) is impaired in cya1 and sycrp1
mutants. The fact that phase 3 motility can be restored to WT
levels, simply by the addition of cAMP to the cya1 mutants, in
all light conditions tested indicates that cAMP levels are crit-
ical for phase 3 phototaxis. A combined effect of glucose,
cAMP, and light is probably involved in the final output that
controls the complex phototactic pathway (3, 25).
FIG. 6. Representative examples of movement tracks of WT cells and cya1 mutants in white light. WT cells (left) and cya1 mutant cells (middle)
were spotted on 0.4% agarose BG-11–10 mM glucose motility plates under directional white light for 24 h. We identified single cells (since groups
of cells are not possible to track), tracked cell movements for 30 min by time-lapse video microscopy, and quantified them with Metamorph tracking
software. A composite made up of randomly selected individual movement tracks was copied and combined into a single figure. The direction of
the white light source is shown by an arrow. Bar, 100 ?m. The panel on the right shows the path length for a particular track from start to finish
and the displacement which is calculated from the start to the finish points.
VOL. 188, 2006NOTES7309

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This work was supported by grant no. 0110544 from the National
Science Foundation (D.B.) and the Carnegie Institution.
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