The CcrM DNA methyltransferase is widespread in the alpha subdivision of proteobacteria, and its essential functions are conserved in Rhizobium meliloti and Caulobacter crescentus

Abstract

The Caulobacter crescentus DNA methyltransferase CcrM (M.CcrMI) methylates the adenine residue in the sequence GANTC. The CcrM DNA methyltransferase is essential for viability, but it does not appear to be part of a DNA restriction-modification system. CcrM homologs are widespread in the alpha subdivision of gramnegative bacteria. We have amplified and sequenced a 258-bp region of the ccrM gene from several of these bacteria, including Rhizobium meliloti, Brucella abortus, Agrobacterium tumefaciens, and Rhodobacter capsulatus. Alignment of the deduced amino acid sequences revealed that these proteins constitute a highly conserved DNA methyltransferase family. Isolation of the full-length ccrM genes from the aquatic bacterium C. crescentus, the soil bacterium R. meliloti, and the intracellular pathogen B. abortus showed that this sequence conservation extends over the entire protein. In at least two alpha subdivision bacteria, R. meliloti and C. crescentus, CcrMmediated methylation has important cellular functions. In both organisms, CcrM is essential for viability. Overexpression of CcrM in either bacterium results in defects in cell division and cell morphology and in the initiation of DNA replication. Finally, the C. crescentus and R. meliloti ccrM genes are functionally interchangeable, as the complemented strains are viable and the chromosomes are methylated. Thus, in both R. meliloti and C. crescentus, CcrM methylation is an integral component of the cell cycle. We speculate that CcrM-mediated DNA methylation is likely to have similar roles among alpha subdivision bacteria.DNA methylation has been identified in both prokaryotes and eukaryotes and has been implicated in many critical cellular processes, including transcriptional regulation (4, 40), initiation of DNA replication (3, 6, 28) and genomic imprinting (33, 34). In mammals and plants, DNA methylation is essential for development. Mice with a null mutation in a cytosine DNA methyltransferase die as embryos (19), and reduced cytosine methylation in Arabidopsis thaliana results in major defects in vegetative and reproductive development (12,35). In prokaryotes, DNA methylation is largely associated with DNA restriction-modification (R/M) systems, where its main function is to allow the cell to differentiate between self and foreign DNA (2, 41). However, bacterial DNA methylation does have other significant biological roles. In Escherichia coli, the Dam DNA methyltransferase (see reference 25 for a recent review), which is not part of an R/M system, regulates several cellular processes, including mismatch repair (15, 24), control of initiation of DNA replication (3,20), and the regulation of gene expression (30,40). Although Dam methylation is involved in a variety of important physiological functions, it is not essential for viability (23). The E. coli Dam methyltransferase is present in several related enteric bacteria, where its function is probably conserved (1).In the bacterium Caulobacter crescentus, the CcrM (M.CcrMI) DNA methyltransferase methyl...

Global analysis of cell cycle gene expression of the legume symbiontSinorhizobium meliloti

“…Another interesting deviation from the C. crescentus cell cycle regulatory network is the absence of CtrA-binding motifs upstream of the gene encoding the essential cell cycle-regulated methylase, CcrM (12,22). In C. crescentus, CcrM methylates newly replicated DNA at the adenine of GANTC site and the activation of ccrM transcription by CtrA at the end of S phase results in the remethylation of the chromosome (45).…”

“…5A). Previous analysis identified GANTC sites upstream of ccrM, suggesting a method of autoregulation, but it is still unclear how transcription of ccrM is up-regulated after completion of S phase in S. meliloti (12).…”

“…The results of this study suggest that the S. meliloti cell cycle transcriptional regulatory network, especially the regulon of the master regulator CtrA, has adapted specifically to fit its lifestyle both within the soil and its legume host. are essential and/or functionally conserved in S. meliloti, our understanding of the role of transcriptional regulation in the S. meliloti cell cycle and the link between the cell cycle and cellular differentiation during symbiosis has been limited (11,12,24,25).…”

“…For example, it has been shown that altering the expression of genes central to S. meliloti cell cycle processes (i.e., ftsZ, dnaA, minE, and ccrM) produces bacteroid-like polyploid cells (10)(11)(12)(13) and that mutation of conserved cell cycle regulators (cbrA, cpdR1, and divJ) blocks bacteroid formation and symbiosis (14)(15)(16). Furthermore, in the α-proteobacterium Caulobacter crescentus, the cellular differentiation program governing morphological and replicative asymmetry in progeny cells is genetically integrated with the cell cycle (17).…”

Fun stories about Brucella: the “furtive nasty bug”

“…In the past few years, ccrM and/or ctrA homologues have been identi®ed in many aProteobacteria including S. meliloti, Agrobacterium tumefaciens, Rhodobacter capsulatus and B. abortus (Stephens et al, 1996;Wright et al, 1997;Lang and Beatty, 2000;Lestrate et al, 2000;Robertson et al, 2000;Barnett et al, 2001a,b;Kahng and Shapiro, 2001;Bellefontaine et al, 2002). The conservation of both ctrA and ccrM in these organisms, displaying very distinct lifecycles and ecology (free-living, symbiotic or parasitic bacteria, symmetric or asymmetric cell division, motile or non-motile, single or multiple replicas) raises the question of their functions.…”