DpnA,amethylase forsingle-strand DNA intheDpnIIrestriction system, anditsbiological function

Communicated byHamilton 0.Smith, September 11,1989(received forreview July 5,1989) ABSTRACT ThetwoDNA-adenine methylases encoded by theDpnIIrestriction genecassette werepurified, andtheir activities werecompared onvarious DNA substrates. DpnA wasabletomethylate single-strand DNA anddouble-strand DNA,whereas DpnMmethylated onlydouble-strand DNA. Although bothenzymes actat5'-GATC-3' inDNA,DpnAcan also methylate sequences altered intheguanine position, butat a lowerrate. A deletion mutation inthedpnAgenewas constructed andtransferred tothechromosome. Transmission bywayofthetransformation pathway ofmethylated and unmethylated plasmids todpnAmutant andwild-type recipi- ents wasexamined. Themutant cells restricted unmethylated donorplasmid establishment muchmorestrongly thandid wild-type cells. Inthewildtype, thesingle strands ofdonor plasmid DNA thatenter bythetransformation pathway are apparently methylated byDpnAprior toconversion ofthe plasmid toadouble-strand form, inwhich theplasmid would besusceptible totheDpnIIendonuclease. Thebiological function ofDpnAmay,therefore, betheenhancement of plasmid transfer toDpnTI-containing strains ofStreptococcus pneumoniae. TheDpnIandDpnIIrestriction systems arefoundin