Cytosine DNA methylation influences drug resistance in Escherichia coli through increased sugE expression.

Escherichia coli K-12 strains contain the orphan cytosine-5 DNA methyltransferase enzyme Dcm (DNA cytosine methyltransferase). Two recent reports indicate that Dcm has an influence on stationary phase gene expression in E. coli. Herein, we demonstrate that dcm knockout cells overexpress the drug resistance transporter SugE, which has been linked to ethidium bromide (ETBR) resistance. SugE expression also increased in the presence of the DNA methylation inhibitor 5-azacytidine, suggesting that Dcm-mediated DNA methylation normally represses sugE expression. The effect of Dcm on sugE expression is primarily restricted to early stationary phase, and RpoS is required for robust sugE expression. Dcm knockout cells are more resistant to ETBR than wild-type cells, and complementation with a plasmid-borne dcm gene restores ETBR sensitivity. SugE knockout cells are more sensitive to ETBR than wild-type cells. These data indicate that Dcm influences the sensitivity to an antimicrobial compound through changes in gene expression.