Abstract
Virtually every organism so far tested has been found to possess an extremely efficient DNA repalr mechanism to ensure that certaln alkylated oxygens do not accumulate in the genome. The repalr is executed by DNA methyltransferases (MTases) which repalr DNA O6‐methylguanine (O6MeG), O4‐methylthymine (O4MeT) and methylphosphotriesters (MePT). The mechanism is rather extravagant because an entire protein molecule is expended for the repalr of just one, or sometimes two, O‐alkyl DNA adduct(s). Cells profit from such an expensive transaction by earning protection agalnst death and mutation by alkylating agents. This review considers the structure, function and biological roles of a number of well‐characterized microbial DNA repalr MTases.