Contrasting lifestyles of rolling-circle phages and plasmids

The rolling-circle mechanism of DNA replication is used by small prokaryoti c genomes, such as single-stranded phages and plasmids. However, phages and plasmids have adapted the rolling-circle mechanism differently to suit the ir contrasting biological needs. The phi X174 phage uses a monomeric initia tor protein catalytically, displays incomplete termination and recycles the initiator protein, in order to mass-produce phage progeny. By contrast, to control replication precisely, the pT181 plasmid uses a dimeric initiator protein stochiometrically, completes termination and inactivates the initia tor after each replication cycle. The phi X174 phage and the pT181 plasmid represent paradigmatic adaptations of the rolling-circle mechanism and coul d provide models for other replicons.