A mutant of Bacillus subtilis defective in the constitutive activity of 
O6-alkylguanine-DNA alkyltransferase was isolated from a strain (ada-1) 
deficient in the adaptive response to DNA alkylation. Cells carrying the 
mutation dat-1 which was responsible for the defect in constitutive activity 
exhibited hypersensitivity for lethality and mutagenesis when challenged with 
methyl-nitroso compounds. The constitutive activity is independent of the 
adaptive response, and seems to function as a basal defense against 
environmental alkylating agents.