Bacterial genomes can be methylated at particular motifs by methyltransferases (MTs). This DNA modification allows restriction endonucleases (REs) to discriminate between self and foreign DNA. While the accepted primary function of such restriction modification (RM) systems is to degrade incoming foreign DNA, other roles of RM systems and lone RE or MT components have been found in genome protection, stability, and the regulation of various phenotypes. The Burkholderia cepacia complex (Bcc) is a group of closely related opportunistic pathogens with biotechnological potential. Here, we constructed and analyzed mutants lacking various RM components in the clinical Bcc isolate Burkholderia cenocepacia H111 and used single-molecule, real-time (SMRT) sequencing of single mutants to assign the B. cenocepacia H111 MTs to their cognate motifs. DNA methylation is shown to affect biofilm formation, cell shape, motility, siderophore production, and membrane vesicle production. Moreover, DNA methylation had a large effect on the maintenance of the Bcc virulence megaplasmid pC3. Our data also suggest that the gp51 MT-encoding gene, which is essential in H111 and is located within a prophage, is required for maintaining the bacteriophage in a lysogenic state, thereby ensuring a constant, low level of phage production within the bacterial population. IMPORTANCE While the genome sequence determines an organism's proteins, methylation of the nucleotides themselves can confer additional properties. In bacteria, MTs modify specific nucleotide motifs to allow discrimination of "self" from "nonself" DNA, e.g., from bacteriophages. Restriction enzymes detect "nonself" methylation patterns and cut foreign DNA. Furthermore, methylation of promoter regions can influence gene expression and hence affect various phenotypes. In this study, we determined the methylated motifs of four strains from the Burkholderia cepacia complex of opportunistic pathogens. We deleted all genes encoding the restriction and modification components in one of these strains, Burkholderia cenocepacia H111. It is shown that DNA methylation affects various phenotypic traits, the most noteworthy being lysogenicity of a bacteriophage and maintenance of a virulence megaplasmid.
Keywords: DNA methylation; bacterial epigenetics; restriction-modification systems.