In Vivo Replication of Filamentous Phage DNA

Intracellular Forms of Filamentous Phage DNA The first step in the replication of filamentous phage DNA is the conversion of the infecting viral DNA to a circular, duplex replicative form (RF). This RF molecule, termed the parental RF, replicates to produce a pool of progeny RF molecules. The accumulated RF molecules then serve both as templates for transcription of viral genes and as a source of progeny single-stranded (SS) DNA. The latter molecules are produced later in the life cycle by an asymmetric replication process in which the complementary strand of an RF molecule serves as a stable, circular template for the repeated displacement of viral single strands. The duplex replicative form occurs most often as a covalently closed, superhelical DNA (RFI). Infected cells also contain small, but significant, amounts of nicked circular RF molecules which contain one or more single-strand discontinuities (RFII) and relaxed circles in which both strands are covalently closed (RFIV), RFIII, a unit-length, linear RF, appears to occur only as an artifact. Miniature forms of RF and SS DNA are observed in cells infected with phage preparations containing “miniphage.” Such particles were first detected in phage preparations obtained after multiple passages beyond the original single-plaque isolation (Griffith and Kornberg 1974; Enea and Zinder 1975). These phage have extensive deletions of the genome and, consequently, do not contain any intact genes. Their replication and morphogenesis are dependent on gene functions provided by a helper phage. The miniphages contain only the region around the origin of replication and...