DNA methylation affects gene expression but not chromatin structure in Escherichia coli

The activity of DNA adenine methyltransferase (Dam) and DNA cytosine methyltransferase (Dcm) together account for nearly all methylated nucleotides in the Escherichia coli K-12 MG1655 genome. Previous studies have shown that perturbation of DNA methylation alters E. coli global gene expression, but it is unclear whether the methylation state of Dam or Dcm target sites regulates local transcription. We observed an underrepresentation of Dam sites in transcriptionally silent extended protein occupancy domains (EPODs), and we thus hypothesized that a methylation-deficient version of MG1655 would show large-scale aberrations in chromatin structure. To test our hypothesis, we cloned methyltransferase deletion strains and performed global protein occupancy profiling using high resolution in vivo protein occupancy display (IPOD-HR), chromatin immunoprecipitation for RNA polymerase (RNAP-ChIP), and transcriptome abundance profiling using RNA-Seq. Our results indicate that loss of DNA methylation does not result in large-scale changes in genomic protein occupancy such as the formation of EPODs. However, loci with dense clustering of Dam methylation sites show methylation-dependent changes in local RNA polymerase and total protein occupancy, but local transcription is unaffected. Our transcriptome profiling data indicates that deletion of dam and/or dcm results in significant expression changes within some functional gene categories including SOS response, flagellar synthesis, and translation, but these expression changes appear to result from indirect regulatory consequences of methyltransferase deletion. In agreement with the downregulation of genes involved in flagellar synthesis, dam deletion is characterized by a swimming motility-deficient phenotype. We thus conclude that DNA methylation does not control the overall protein occupancy landscape of the E. coli genome, and that observable changes in gene regulation are generally not resulting from regulatory consequences of local methylation state. Overall design: Three biological replicates each of wild-type, ?dam, ?dcm, and ?dam?dcm E. coli K12 MG1655 were grown in MOPS Rich Defined Media (RDM) with glucose to an OD600 of .2, at which point an aliquot was taken for RNA-seq processing, and the main culture was then treated with rifampicin, crosslinked with formaldehyde, and then harvested. Each biological replicate was then split to perform IPOD interphase extraction, ChIP for RNA polymerase, and input processing.