DNA methylation by CcrM contributes to genome maintenance in theAgrobacterium tumefaciensplant pathogen

ABSTRACT The c ell c ycle-regulated DNA m ethyltransferase CcrM is conserved in most Alphaproteobacteria , but its role in bacteria with complex or multicentric genomes remains unexplored. Here, we compare the methylome, the transcriptome and the phenotypes of wild-type and CcrM-depleted Agrobacterium tumefaciens cells with a dicentric genome with two essential replication origins. We find that DNA methylation has a pleiotropic impact on motility, biofilm formation and viability. Remarkably, CcrM promotes the expression of the repABC Ch2 operon, encoding proteins required for replication initiation/partitioning at ori2 , and inhibits gcrA , encoding a conserved g lobal c ell cycle regulator. Imaging ori1 and ori2 in live cells, we show that replication from ori2 is often delayed in cells with a hypo-methylated genome, while ori2 over-initiates in cells with a hyper-methylated genome. We thus propose that methylation by CcrM stimulates RepABC-dependent chromosomal origins, uncovering a novel and original connection between CcrM-dependent DNA methylation and genome maintenance in an Alphaproteobacterial pathogen.