3-Methyladenine residues in DNA induce the SOS function sfiA in Escherichia coli.

The induction by methylating agents of the SOS function sfiA was measured by means of a sfiA::lac operon fusion in Escherichia coli mutants defective in alkylation repair. The sfiA operon was turned on at a 10-fold lower concentration of methylmethane sulfonate or dimethyl sulfate in tagA strains, lacking specific 3-methyladenine-DNA glycosylase, than in wild-type strains. In contrast, the induction of sfiA by u.v. light was not affected by a tagA mutation. We confirm that tagA strains specifically accumulate 3-methyladenine in their DNA. We conclude that the persistence of 3-methyladenine in E. coli DNA most likely induces the SOS functions. Results on in vitro DNA synthesis further suggest that this induction is due to an unscheduled arrest of DNA synthesis at this lesion.