Identification of a methyl-specific restriction system mediated by a conjugative element from Streptomyces bambergiensis

pBL2 was identified genetically but not physically in Streptomyces lividans after its mating with S. bambergiensis. During conjugation, pBL2 was transferred at high frequency to S. lividans and S. coelicolor. pBL2.1 DNA isolated from S. coelicolor exconjugants as a circular plasmid was shown to derive from the genome of S. bambergiensis. S. lividans carrying pBL2 or pBL2.1 acquired a methyl-specific restriction (MsrA ⁺) phenotype. The corresponding enzyme was partially purified and shown to resemble a class II endonuclease which cleaves Dam-methylated DNA preferentially.