Mutations that eliminate the requirement for the vertex protein in bacteriophage T4 capsid assembly

The capsid of bacteriophage T4 is composed of two essential structural proteins, gp23, the major constituent of the capsid, and gp24, a less prevalent protein that is located in the pentameric vertices of the capsid. gp24 is required both to stabilize the capsid and to allow it to be further matured. This requirement can be eliminated by bypass-24 (byp24) mutations within g23. We have isolated, cloned and sequenced several new byp24 mutations. These mutations are cold-sensitive in the absence of gp24, and are located in regions of g23 not known to contain any other mutations affecting capsid assembly. The cold-sensitivity of the byp24 mutations can be reduced by further mutations within g23 (trb mutations). Cloning and sequencing of these trb mutations has revealed that they lie in regions of g23 that contain clusters of mutations that cause the production of high levels of petite and giant phage (ptg mutations). Despite the proximity of the trb mutations to the ptg mutations, none of the ptg mutations has a Trb phenotype. The mutation ptE920g, which is also located near one of the ptg clusters, and which produces only petite and wild-type phage, has been shown to confer a Trb but not a Byp24 phenotype. The relevance of these observations to our understanding of capsid assembly is discussed.