Loretta Thomas, Chul-Hak Yang, and David A. Goldthwait* abstract: Two enzymes have been partially purified from Escherichia coli and designated 3-methyladenine DNA gly-cosylases I and II. The glycosylase I is that described by Riazuddin & Lindahl [Riazuddin, S., & Lindahl, T.(1978) Biochemistry 17, 2110-2118]. The apparent molecular weight of glycosylase I is 20000, and that of II is 27 000. Glycosylase I releases 3-methyladenine (3-MeA) while II releases 3-MeA, 3-methylguanine (3-MeG), 7-methylguanine (7-MeG), and 7-methyladenine (7-MeA). Therate of release of 3-MeA by glycosylase II is 30 times thatof 7-MeG. Glycosylase I is missing in mutants tag 1 and tag 2 [Karran, P., Lindahl, T., Ofsteng, I., Evenson, GB, & Seeberg, E.(1980) J. Mol. Biol. 140, 101-127], In crude extracts, the 3-MeA activity of II is approximately 10% of the total 3-MeA activity. A 50%

The first evidence for an enzyme that recognizes alkylated DNA was presented by Strauss (1962), who demonstrated that extracts of Micrococcus lysodeikticus and Bacillus subtilis inactivated transforming DNA treated with methyl meth-anesulfonate (MMS). Reiter etal.(1967) demonstrated that an endonuclease is present in extracts of B. subtilis that rec-ognizes DNA treated with MMS and that there is repair in vivo of the bacterial DNA after treatment with the alkylating agent. An enzyme fraction from Escherichia coli active on