Wright R, Stephens C, Shapiro L
Department of Developmental Biology, Stanford University, California 94305-5427, USA.
J Bacteriol. 1997 Sep;179(18):5869-77. doi: 10.1128/jb.179.18.5869-5877.1997.
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 gram-negative bacteria. We have amplified and sequenced a 258-bp region of the cerM 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, CcrM-mediated 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.
新月柄杆菌DNA甲基转移酶CcrM（M.CcrMI）可使序列GANTC中的腺嘌呤残基发生甲基化。CcrM DNA甲基转移酶对于细菌存活至关重要，但它似乎并非DNA限制修饰系统的一部分。CcrM同源物广泛存在于革兰氏阴性菌的α亚群中。我们从几种这类细菌中扩增并测序了cerM基因的一个258bp区域，这些细菌包括苜蓿根瘤菌、流产布鲁氏菌、根癌土壤杆菌和荚膜红细菌。推导的氨基酸序列比对显示，这些蛋白质构成了一个高度保守的DNA甲基转移酶家族。从水生细菌新月柄杆菌、土壤细菌苜蓿根瘤菌和细胞内病原体流产布鲁氏菌中分离出全长ccrM基因，结果表明这种序列保守性在整个蛋白质中都存在。在至少两种α亚群细菌苜蓿根瘤菌和新月柄杆菌中，CcrM介导的甲基化具有重要的细胞功能。在这两种生物中，CcrM对存活都是必不可少的。在任何一种细菌中过表达CcrM都会导致细胞分裂、细胞形态以及DNA复制起始出现缺陷。最后，新月柄杆菌和苜蓿根瘤菌的ccrM基因在功能上是可互换的，因为互补菌株能够存活且染色体发生了甲基化。因此，在苜蓿根瘤菌和新月柄杆菌中，CcrM甲基化都是细胞周期的一个组成部分。我们推测CcrM介导的DNA甲基化在α亚群细菌中可能具有类似作用。
