A CELL-CYCLE-REGULATED BACTERIAL-DNA METHYLTRANSFERASE IS ESSENTIAL FOR VIABILITY

\The CcrM adenine DNA methyltransferase, which specifically modifies G ANTC sequences, is necessary for viability in Caulobacter crescentus, To our knowledge, this is the first example of an essential prokaryoti c DNA methyltransferase that Is not part of a DNA restriction/modifica tion system, Homologs of CcrM are widespread in the alpha subdivision of the Proteobacteria, suggesting that methylation at GANTC sites may have important functions in other members of this diverse group as wel l, Temporal control of UNA methylation state has an important role in Caulobacter development, and we show that this organism utilizes an un usual mechanism for control of remethylation of newly replicated DNA, CcrM is synthesized de novo late in the cell cycle, coincident with fu ll methylation of the chromosome, and is then subjected to proteolysis prior to cell division.