Dam-dependent phase variation of Ag43 in Escherichia coli is altered in a seqA mutant
ABSTRACT In Escherichia coli, phase variation of the outer membrane protein Ag43 encoded by the agn43 gene is mediated by DNA methylation and the global regulator OxyR. Transcription of agn43 occurs (ON phase) when three Dam target sequences in the agn43 regulatory region are methylated, which prevents the repressor OxyR from binding. Conversely, transcription is repressed (OFF) when these Dam target sequences are unmethylated and OxyR binds. A change in expression phase requires a concomitant change in the DNA methylation state of these Dam target sequences. To gain insight into the process of inheritance of the expression phase and the DNA methylation state, protein-DNA interactions at agn43 were examined. Binding of OxyR at agn43 was sufficient to protect the three GATC sequences contained within its binding site from Dam-dependent methylation in vitro, suggesting that no other factors are required to maintain the unmethylated state and OFF phase. To maintain the methylated state of the ON phase, however, Dam must access the hemimethylated agn43 region after DNA replication, and OxyR binding must not occur. OxyR bound hemimethylated agn43 DNA, but the affinity was severalfold lower than for unmethylated DNA. This presumably contributes to the maintenance of the methylated state but, at the same time, may allow for infrequent OxyR binding and a switch to the OFF phase. Hemimethylated agn43 DNA was also a binding substrate for the sequestration protein SeqA. Thus, SeqA, OxyR and Dam may compete for the same hemimethylated agn43 DNA that is formed after DNA replication in an ON phase cell. In isolates with a mutant seqA allele, agn43 phase variation rates were altered and resulted in a bias to the OFF phase. In part, this can be attributed to the observed decrease in the level of DNA methylation in the seqA mutant.
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ABSTRACT: The successful inheritance of genetic information across generations is a complex process requiring replication of the genome and its faithful segregation into two daughter cells. At each replication cycle there is a risk that new DNA strands incorporate genetic changes caused by miscopying of parental information. By contrast the parental strands retain the original information. This raises the intriguing possibility that specific cell lineages might inherit "immortal" parental DNA strands via non-random segregation. If so, this requires an understanding of the mechanisms of non-random segregation. Here we review several aspects of asymmetry in the very symmetrical cell, Escherichia coli, in the interest of exploring the potential basis for non-random segregation of leading- and lagging-strand replicated chromosome arms. These considerations lead us to propose a model for DNA replication that integrates chromosome segregation and genomic localisation with non-random strand segregation.Seminars in Cell and Developmental Biology 05/2013; DOI:10.1016/j.semcdb.2013.05.010 · 5.97 Impact Factor
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ABSTRACT: Epigenetic modification plays a dual role in bacterial infection. On the one hand, the expression of cer- tain genes in the bacterial organism is regulated by epigenetic mechanisms. For example, the expression of components of the type IV secretion system in Gram-negative bacteria like enteroaggregative Esche- richia coli (EAggEC), which is responsible for the secretion of proteins involved in host cell invasion and bacterial killing, is regulated by DNA methylation (Brunet et al., 2011). Differences in the digestion-pattern of endonucleases recognizing methylated adenine at 5’- GATC-3’-sites in the genomes of several Gram-negative bacterial species like Campylobacter coli (Wright et al., 2010) and Campylobacter jejuni (Gu et al., 2009) suggest that further genes are regulated by epigenetic modifications as well. Bioinformatic investigations demonstrated epigenetic regulation of type III secretion system (TTSS) associated toxicity in the non-fermentative Gram-negative facultative pathogen Pseudomonas aeruginosa (Filopon et al., 2006). A generalized logical analysis indicated epigenetic control of Pseudo- monas aeruginosa’s mucoidy (Guespin-Michel & Kaufman, 2001). There are even hints for epigenetic regulation of multistationary of bacterial growth (Guespin-Michel & Kaufman, 2001). On the other hand, bacterial infections induce epigenetic changes in the cells of the infected organ- ism. It is a well described phenomenon that viral proteins affect the activity of cellular promoters via epi- genetic mechanisms. In the last years similar effects were demonstrated for bacterial infections. Promotor silencing by hypermethylation at CpG dinucleotides was described for Campylobacter rectus in placenta tissue (Bobetsis et al., 2007) and for Helicobacter pylori in gastric mucosa (Chan et al., 2003). This epi- genetic reprogramming might play a critical role in fetal growth and ontogenetic programming but also in carcinogenesis. In contrast, methylation of bacterial 16S rRNA is a resistance mechanism providing high-level re- sistance to aminoglycosides in actinomycetes, Enterobacteriaceae and Gram-negative, non-fementing, rod-shaped bacteria like Pseudomonas aeruginosa (Yokoyama et al., 2003; Doi et al., 2004; Périchon et al., 2008). However, this phenomenon is no epigenetic mechanism as gene expression is not affected. Infection with Gram-negativ bacteria induces epigenetic modifications not only in the invading bacterial cells, but also in the host tissue. Epigenetic consequences of bacterial infection are important for the understanding of the pathomechanisms and may have principal therapeutic implications.Introduction to Genetics: DNA Methylation, Histone Modification and Gene Regulation, 1st edited by Jun Wan, 07/2013: chapter Epigenetic Modifications in Gram-Negative Bacteria and the Induction of Patho-Epigenetic Modifications in Host Tissue: pages 270; iConcept Press., ISBN: 978-1477554944
Article: DNA Methylation06/2014; 2014(4). DOI:10.1128/ecosalplus.ESP-0003-2013