I am reading the Molecular Biology of the Gene VI. I found that the initiation of genome DNA replication is triggered by the methylation of both double strands of oriC of E. coli. This mismatch repair system relies on the by the methylation of parental strands to recognize the newly synthesized strand. However, I found that there are several commercial E. coli strains with defective methylation genotypes for extraction of plasmid without methylation, in order to use some methylation sensitive endonuclease. How do these E. coli strains with defective methylation genotype survive?
Regarding chromosomal replication, it is only hemimethylated origins that do not work. Completely non-methylated oriC is functional. In the case of mismatch repair, this system is not essential (although dam- strains exhibit about 1000-fold higher mutation rates than their wild-type counterparts, and are more sensitive to UV, etc).
Regarding chromosomal replication, it is only hemimethylated origins that do not work. Completely non-methylated oriC is functional. In the case of mismatch repair, this system is not essential (although dam- strains exhibit about 1000-fold higher mutation rates than their wild-type counterparts, and are more sensitive to UV, etc).
Alejandro is correct, methylation does not initiate replication, it is hemimethylation that inhibits initiation of replication. So any change from the hemi-methylated state (complete methylation or no methylation) would be replication competent. But note there are other events and factors that trigger replication initiation, hemimethylation is only a transient inhibitor.
Very nice question. Well ask the E.coli as to how are they surviving. Well at the onset let me point out that your question of methylating E.coli is not correct. You can not methylate a whole living microorganism. Secondly if you are careful enough in doing your literature survey, there are several reports that methylation is not the only factor by which newly synthesized DNA strand can be differentiated from the parent strand during the mismatch repair system (so called). In fact you can extract plasmids without methylation and then even identify these. I am referring to the plasmids of lambda phages or sometime even omega phages of E.coli.
Nice question and nice answers. Actually I'm reading some articles and reviews about DNA methylation in prokaryotes. I wish to add in some new information: it is speculated that Dam (DNA MTases) may be essential to coordinate DNA replication in bacteria with two or more chromosomes (Casadesús and Low, 2006, in ‘Epigenetic Gene Regulation in the Bacterial World’, page 834, the first parag). E. coli has one chromosome so artificial Dam- strains could be used in the lab. However, DR. Martin is right, the Dam deletion may affect the performance of the modified strains.
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