The DNA Replication Cycle of the Isometric Phages

Abstract

It is just twenty years since Robert Sinsheimer reported the results of his initial studies on a small and previously uncharacterized virus, ϕ X174. It is one of the ironies of research that a virus initially so different and odd should gradually emerge as a system offering some of the deepest insights into a major area of molecular biology, DNA replication. In contrast to the disruptive entrance into the host cell of larger viruses such as T4 and T7, bacteriophage ϕ X and the related isometric phage G4 set to work quietly, not disturbing the host cell except for an eventual cessation of bacterial DNA synthesis (Tessman 1966; Stone 1970). Even in the midst of this seeming order, however, the virus diverts the host sufficiently to generate a thousandfold replication of itself—a process which is readily observed if one studies a mutant virus that does not lyse the host prematurely. The host cell supplies almost all of the enzymatic machinery required for the viral DNA replication cycle. It is precisely for this reason that the single-stranded (SS) DNA phages are able to serve as a window into the DNA replication apparatus of Escherichia coli. The way in which the viral chromosome is processed during its replication reveals the reaction mechanisms of many host enzymes that normally play a role in bacterial DNA synthesis but which are commandeered by the virus after infection. In this article we will consider the various steps in the ϕ X and G4 replication cycles and discuss the...