A Caulobacter DNA methyltransferase that functions only in the predivisional cell

Caulobacter crescentus was found to have a DNA methyltransferase, CcrM, that methylates the adenine base of the HinfI recognition sequence, GANTC. The ccrM gene was cloned, and DNA sequence analysis revealed that the predicted amino acid sequence has 49% identity with the Haemophilus influenzae methyltransferase HinfM. Expression of the ccrM gene was found to be restricted to the portion of the cell cycle immediately prior to cell division. At three separate chromosomal sites the CcrM recognition sequence is fully methylated in swarmer cells, becomes hemimethylated upon DNA replication in stalked cells, and does not become remethylated until just prior to cell division. The time of methyltransferase expression coincides with the time of methylation of these three chromosomal sites and of plasmid DNA in the predivisional cell. When ccrM gene expression is placed under control of a constitutive promoter, these chromosomal sites are fully methylated throughout the cell cycle. A high proportion of morphologically aberrant cells, and cells that have undergone an additional chromosome replication initiation, are found in this population. Thus, the temporal control of this methyltransferase appears to contribute to the accurate cell-cycle control of DNA replication and cellular morphology.