Replication Control in Phage Lambda

Abstract

Early in the productive growth cycle following infection, λ DNA can be found in doubly branched circular structures (Ogawa et al., 1968). The positions of branch points in these molecules suggest that each molecule carries two replication forks moving away from a locus in the region of genes O and P (Schnos and Inman, 1970). These suggestions have been substantiated and refined by measurements of replication in deletion prophages in the presence of helper (Stevens et al., this volume). The replication process can be divided into a step creating a replication fork (initiation) and the subsequent movement of the fork along the DNA template (progression or movement). Three λ genes ( N , O , and P ) are known to specify products that enter into the replication process. The host supplies the biosynthetic machinery for DNA precursors, and at least one gene product that is involved directly in replication of both the host chromosome and λ DNA beyond the precursor level (Hirota et al., 1968; Fangman and Feiss, 1969; Georgopoulos and Herskowitz, this volume). When λ DNA is repressed, it does not replicate autonomously (Wolf and Meselson, 1963), even if the products of genes N , O, and P are supplied by a heteroimmune hybrid of λ (Thomas and Bertani, 1964; Russo et al., 1970). Repressor not only prevents the synthesis of the products of genes N, O, and P ; it also blocks their action. The latter aspect of repression is to be called an epistatic block. A mutant, t 11, which is defective in...