The CcrM DNA methyltransferase of Agrobacterium tumefaciens is essential, and its activity is cell cycle regulated

DNA methylation is now recognized as a regulator of multiple bacterial cell ular processes. CcrM is a DNA adenine methyltransferase found in the alpha subdivision of the proteobacteria, Like the Dam enzyme, which is found prim arily in Escherichia coli and other gamma proteobacteria, it does not appea r to be part of a DNA restriction-modification system. The CcrM homolog of Agrobacterium tumefaciens was found to be essential for viability, Overexpr ession of CcrM is associated with significant abnormalities of cell morphol ogy and DNA ploidy, Mapping of the transcriptional start site revealed a co nserved binding motif for the global response regulator CtrA, at the -35 po sition; this motif was footprinted by purified Caulobacter crescentus CtrA protein in its phosphorylated state. We have succeeded in isolating synchro nized populations of Agrobacterium cells and analyzing their progression th rough the cell cycle. We demonstrate that DNA replication and cell division can be followed in an orderly manner and that flagellin expression is cycl ic, consistent with our observation that motility varies during the cell cy cle. Using these synchronized populations, we show that CcrM methylation of the chromosome is restricted to the late S phase of the cell cycle. Thus, within the alpha subdivision, there is a conserved cell cycle dependence an d regulatory mechanism controlling ccrM expression.