The Intergenic Region and the Origins for Filamentous Phage DNA Replication

Phages that have a single-stranded genome, such as ϕX174 and fd, begin their life cycles in infected cells with the conversion of their viral circular DNA into the covalently closed, duplex, parental replicative form (RF) DNA (stage I, single-stranded to double-stranded SS→DS DNA replication). The next stage of growth requires a phage-coded protein that will nick the RF DNA at the origin and prepare it as a substrate for further replication by host proteins (stage II, DS→DS DNA replication). In the last stage, phage proteins complex the viral strand, restrict the formation of complementary strands, and allow the assembly of progeny virus containing the viral DNA circle (stage III, DS→SS DNA replication).