Assembly of bacteriophage phi X174: identification of a virion capsid precursor and proposal of a model for the functions of bacteriophage gene products during morphogenesis.

A capsomeric structure sedimenting with an S value of 108 in sucrose gradients was isolated from Escherichia coli infected with bacteriophage phi X174. The 108S material contained viral proteins F, G, H, and D, and the relative amounts of these proteins in the 108S material were similar to those in the infectious 132S particle, which has previously been described as a possible intermediate in the assembly of 114S phage particles. Electron micrographs indicated that the size and shape of the 108S material resemble those of the 132S particle. The 108S material contained no DNA, and its formation occurred independently of DNA synthesis. The 108S material accumulated in infected cells when viral DNA replication was prevented either by mutation in phage genes A or C or by removal of thymidine from a culture infected with wild-type phage or with a lysis gene E mutant. Upon restoration of thymidine to cells infected with the lysis gene E mutant and then starved of thymidine, the accumulated 108S material was converted to 132S particles and to 114S phage particles, implying that the 108S material is a precursor of phage particles. A model that proposes possible functions for the products of phi X174 genes A, B, C, D, F, and G during viral replication and phage maturation is described.