title: Genomewide Dam Methylation inEscherichia coliduring Long-Term Stationary Phase.
abstract: DNA methylation in prokaryotes is widespread. The most common modification of the genome is the methylation of adenine at the N-6 position. InEscherichia coliK-12 and many gammaproteobacteria, this modification is catalyzed by DNA adenine methyltransferase (Dam) at the GATC consensus sequence and is known to modulate cellular processes including transcriptional regulation of gene expression, initiation of chromosomal replication, and DNA mismatch repair. While studies thus far have focused on the motifs associated with methylated adenine (meA), the frequency of meA across the genome, and temporal dynamics during early periods of incubation, here we conduct the first study on the temporal dynamics of adenine methylation inE. coliby Dam throughout all five phases of the bacterial life cycle in the laboratory. Using single-molecule real-time sequencing, we show that virtually all GATC sites are significantly methylated over time; nearly complete methylation of the chromosome was confirmed by mass spectroscopy analysis. However, we also detect 66 sites whose methylation patterns change significantly over time within a population, including three sites associated with sialic acid transport and catabolism, suggesting a potential role for Dam regulation of these genes; differential expression of this subset of genes was confirmed by quantitative real-time PCR. Further, we show significant growth defects of thedammutant during long-term stationary phase (LTSP). Together these data suggest that the cell places a high premium on fully methylating the chromosome and that alterations in methylation patterns may have significant impact on patterns of transcription, maintenance of genetic fidelity, and cell survival.IMPORTANCEWhile it has been shown that methylation remains relatively constant into early stationary phase ofE. coli, this study goes further through death phase and long-term stationary phase, a unique time in the bacterial life cycle due to nutrient limitation and strong selection for mutants with increased fitness. The absence of methylation at GATC sites can influence the mutation frequency within a population due to aberrant mismatch repair. Therefore, it is important to investigate the methylation status of GATC sites in an environment where cells may not prioritize methylation of the chromosome. This study demonstrates that chromosome methylation remains a priority even under conditions of nutrient limitation, indicating that continuous methylation at GATC sites could be under positive selection.
authors.affiliation: Molecular and Computational Biology Section, Department of Biological Sciences, University of Southern California, Los Angeles, California, USA.
authors.forename: Lacey L
authors.initials: LL
authors.lastname: Westphal
authors.affiliation: Molecular and Computational Biology Section, Department of Biological Sciences, University of Southern California, Los Angeles, California, USA.
authors.forename: Peter
authors.initials: P
authors.lastname: Sauvey
authors.affiliation: Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana, USA.
authors.forename: Matthew M
authors.initials: MM
authors.lastname: Champion
authors.affiliation: Molecular and Computational Biology Section, Department of Biological Sciences, University of Southern California, Los Angeles, California, USA.
authors.forename: Ian M
authors.initials: IM
authors.lastname: Ehrenreich
authors.affiliation: Molecular and Computational Biology Section, Department of Biological Sciences, University of Southern California, Los Angeles, California, USA.
authors.forename: Steven E
authors.initials: SE
authors.lastname: Finkel
keywords: Dam methyltransferase; SMRT; epigenetics; long-term stationary phase; long-term survival; methylation