Escherichia coli/Methylation

• Type I methylation systems:
  • E. coli K-12 restricts DNA which is not protected by adenine methylation at sites AA^*C[N₆]GTGC or GCA^*C[N₆]GTT, encoded by the hsdRMS genes(EcoKI). Deletions in these genes removes either the restriction or methylation or both of these functions.
  • E. coli B derivative strains contain an hsdRMS system (EcoBI) restricting and protectiing the sequence TGA^*[N₈]TGCT or AGCA^*[N₈]TCA.
• The mcrA gene (carried on the e14 prophage) restricts DNA which is methylated in C^mCWGG or ^mCG sequences (methylation by the dcm gene product).
• The mcrBC genes restrict R^mC sequences.
• The mrr gene product restricts adenine methylated sequences at CAG or GAC sites.
• E. coli methylates the adenine in GATC (and the corresponding A on the opposite strand) with the dam gene product.
• M.EcoKII methylates the first A at the palindromic site ATGCAT (as well as the corresponding A on the opposite strand), see (Kossykh VG (2004) J. Bact 186: 2061-2067 PMID 15028690) Note that this article has been retracted; the retraction appears to center on textual plagarism, not experimental results. The homology to AvaIII is real. I think I believe it. tk 20:28, 9 December 2005 (EST). Rich Roberts reports: "We have tried ourselves to detect activity with this gene product and cannot detect any methyltransferase activity. In our case we used antibodies able to detect N6-methyladenine or N4 methylcytosine in DNA. The ones we have are very sensitive and should have been able to detect 5 methyl groups in the whole E. coli chromosome. Nothing was detected in an over expressing strain."
• For additional information see E. coli restriction-modification system and the NEB technical information on methylation.