Initiation mechanisms in replication of filamentous phage DNA

Filamentous phage DNA replication occurs in two steps. First, an RNA-primed minus strand is made on the plus strand. Then, a new plus strand is made on the resulting double strand. The RNA primer which initiates synthesis of the minus strand is produced at a specific site on the plus strand template by the host RNA polymerase holoenzyme. Despite a lack of sequences similar to the promoter consensus, the DNA replication origin has a much higher affinity for the holoenzyme than the transcriptional promoters. The non-template strand of the single-stranded −10 region of the origin appears to be responsible for this high affinity. The recognition mechanisms seem to share common features with those in transcriptional promoters. Plus-strand synthesis is initiated by a specific nick introduced into the negatively supercoiled replicative form by the phage-encoded initiator protein. The nicking reaction is preceded by an ordered series of protein-induced DNA conformational changes.