Expression of the site-specific adenine methylase HhaII (GmeANTC, where me is methyl) or PstI (CTGCmeAG) induced the SOS DNA repair response in Escherichia coli. In contrast, expression of methylases indigenous to E. coli either did not induce SOS (EcoRI [GAmeATTC] or induced SOS to a lesser extent (dam [GmeATC]). Recognition of adenine-methylated DNA required the product of a previously undescribed gene, which we named mrr (methylated adenine recognition and restriction). We suggest that mrr encodes an endonuclease that cleaves DNA containing N6-methyladenine and that DNA double-strand breaks induce the SOS response. Cytosine methylases foreign to E. coli (MspI [meCCGG], HaeIII [GGmeCC], BamHI [GGATmeCC], HhaI [GmeCGC], BsuRI [GGmeCC], and M.Spr) also induced SOS, whereas one indigenous to E. coli (EcoRII [CmeCA/TGG]) did not. SOS induction by cytosine methylation required the rglB locus, which encodes an endonuclease that cleaves DNA containing 5-hydroxymethyl- or 5-methylcytosine (E. A. Raleigh and G. Wilson, Proc. Natl. Acad. Sci. USA 83:9070-9074, 1986).