THE PHI-X174-TYPE PRIMOSOME PROMOTES REPLISOME ASSEMBLY AT THE SITE OF RECOMBINATION IN BACTERIOPHAGE-MU TRANSPOSITION

Initiation of Escherichia coli DNA synthesis primed by homologous reco mbination is believed to require the phi X174-type primosome, a mobile priming apparatus assembled without the initiator protein DnaA. We sh ow that this primosome plays an essential role in bacteriophage Mu DNA replication by transposition. Upon promoting transfer of Mu ends to t arget DNA, the Mu transpososome undergoes transition to a pre-replisom e that permits initiation of DNA synthesis only in the presence of pri mosome assembly proteins PriA, DnaT, DnaB and DnaC. These assembly pro teins promote the engagement of primase and DNA polymerase III holoenz yme, initiating semi-discontinuous replication preferentially at the M u left end, The results indicate that these proteins play a crucial ro le in promoting replisome assembly on a recombination intermediate.