A DNA methyltransferase can protect the genome from postdisturbance attack by a restriction-modification gene complex.
In prokaryotic genomes, some DNA methyltransferases form a restriction-modification gene complex, but some others are present by themselves. Dcm gene product, one of these orphan methyltransferases found in Escherichia coli and related bacteria, methylates DNA to generate 5'-C(m)CWGG just as some of its eukaryotic homologues do. Vsr mismatch repair function of an adjacent gene prevents C-to-T mutagenesis enhanced by this methylation but promotes other types of mutation and likely has affected genome evolution. The reason for the existence of the dcm-vsr gene pair has been unclear. Earlier we found that several restriction-modification gene complexes behave selfishly in that their loss from a cell leads to cell killing through restriction attack on the genome. There is also increasing evidence for their potential mobility. EcoRII restriction-modification gene complex recognizes the same sequence as Dcm, and its methyltransferase is phylogenetically related to Dcm. In the present work, we found that stabilization of maintenance of a plasmid by linkage of EcoRII gene complex, likely through postsegregational cell killing, is diminished by dcm function. Disturbance of EcoRII restriction-modification gene complex led to extensive chromosome degradation and severe loss of cell viability. This cell killing was partially suppressed by chromosomal dcm and completely abolished by dcm expressed from a plasmid. Dcm, therefore, can play the role of a "molecular vaccine" by defending the genome against parasitism by a restriction-modification gene complex.
Takahashi
Noriko
N
Division of Molecular Biology, Department of Basic Medical Sciences, Institute of Medical Science, University of Tokyo, Shirokanedai, Japan.
Naito
Yasuhiro
Y
Handa
Naofumi
N
Kobayashi
Ichizo
I
D000818:Animals; D016923:Cell Death; D004248:DNA (Cytosine-5-)-Methyltransferases / Q000378:metabolism*; D019175:DNA Methylation; D015280:DNA Restriction-Modification Enzymes / Q000235:genetics / Q000378:metabolism*; D004269:DNA, Bacterial / Q000378:metabolism; D015252:Deoxyribonucleases, Type II Site-Specific / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development; D016680:Genome, Bacterial; D010802:Phylogeny; D010957:Plasmids
D015280:DNA Restriction-Modification Enzymes; D004269:DNA, Bacterial; D004248:DNA (Cytosine-5-)-Methyltransferases; C046545:CCWGG-specific type II deoxyribonucleases; D015252:Deoxyribonucleases, Type II Site-Specific