Rolling-circle replication of the plasmid pKYM isolated from a gram-negative bacterium

Plasmid pKYM isolated from a Gram-negative bacterium encodes a Rep protein that is essential for plasmid replication. A comparison of Rep protein from pKYM to Rep proteins encoded by other plasmids shows that it has homology to Rep proteins of the pUB110 plasmid family from Gram-positive bacteria. These plasmids replicate by a rolling-circle mechanism in which a tyrosine residue in the Rep protein acts as the acceptor for the 5' end of the single-strand break introduced by the Rep protein. A Tyr----Phe substitution in the pKYM Rep protein abolishes its activity. Strand-specific single-stranded circular plasmid DNA can be recovered from the cells carrying pKYM and thus we propose that the plasmid pKYM replicates by a rolling-circle mechanism.