Chemical adaptation of M. luteus induces repair functions for O-alkylated DNA pyrimidines

Methyl transferases induced during chemical adaptation of M. luteus

Three peaks of methyltransferase activity specific for MNNG alkylated DNA have been identified from extracts of chemically adapted M. luteus. They are designated as TI to TIII in order to their elution from a Sephadex G-75 column. The first one of these peaks has been purified to homogeneity. TI, is an inducible, unusually salt resistant, heat labile protein which corrects O6-methylguanine in alkylated DNA by the transfer of the O6-alkyl group to a cysteine amino acid in the TI protein. There is a stoichiometric relationship between the loss of O6-methylguanine from the DNA and the production of S-methylcysteine. Partially purified TII & TIII proteins show specificity for O4-alkylthymine and methyl phosphotriesters respectively. The mode of repair by the isolated methyltransferases is similar yet there is no competition for substrate specificity. The apparent molecular weights of TI, TII & TIII proteins are 31Kd, 22Kd, and 13Kd respectively.

DNA glycosylase enzymes induced during chemical adaptation of M. luteus

Five peaks of DNA glycosylase activity showing a preference for MNNG alkylated DNA have been identified from extracts of adapted M. luteus. They are numerically designated as GI to GV in order of their decreasing molecular weights. The first two of these peaks have been highly purified. GI, is a constitutive heat labile protein, 35% stimulated by the presence of 50 mM NaCl, acts exclusively on 3 MeA residues in alkylated DNA, 60-70% inhibited by the presence of 2 mM free 3MeA and has been designated as 3MeA DNA glycosylase enzyme. GII, which is an inducible protein, is heat stable, 28% inhibited by the presence of 50 mM NaCl, removes 3MeA, 3MeG, 7MeA & 7MeG with different efficiency, and has been designated as 3,7 methylpurine DNA glycosylase enzyme. The rate of release of 3 methylpurines is 30 times that of 7MeG. There is no activity of either enzyme on O2-MeC, O2-MeT, O4-MeT or O6-MeG. The apparent molecular weights of GI and GII proteins are 28 Kd and 22 Kd respectively.