Zweiger G, Marczynski G, Shapiro L
Department of Genetics, Stanford University School of Medicine, CA 94305.
J Mol Biol. 1994 Jan 14;235(2):472-85. doi: 10.1006/jmbi.1994.1007.
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.
人们发现新月柄杆菌有一种DNA甲基转移酶CcrM，它可使HinfI识别序列GANTC中的腺嘌呤碱基发生甲基化。ccrM基因被克隆出来，DNA序列分析表明，预测的氨基酸序列与流感嗜血杆菌甲基转移酶HinfM有49%的同源性。研究发现ccrM基因的表达仅限于紧接细胞分裂前的细胞周期阶段。在三个不同的染色体位点，CcrM识别序列在游动细胞中完全甲基化，在柄细胞中DNA复制时变成半甲基化，直到细胞分裂前才再次甲基化。甲基转移酶表达的时间与这三个染色体位点以及分裂前细胞中质粒DNA的甲基化时间一致。当ccrM基因表达受组成型启动子控制时，这些染色体位点在整个细胞周期中都完全甲基化。在这个群体中发现了高比例的形态异常细胞以及经历了额外一次染色体复制起始的细胞。因此，这种甲基转移酶的时间控制似乎有助于DNA复制和细胞形态的精确细胞周期控制。
