Host and φX 174 mutations affecting the morphogenesis or stabilization of the 50S complex, a single-stranded DNA synthesizing intermediate

The morphogenetic pathway of bacteriophage φX 174 was investigated in rep mutant hosts that specifically block stage III single-stranded DNA synthesis. The defects conferred by the mutant rep protein most likely affect the formation or stabilization of the 50S complex, a single-stranded DNA synthesizing intermediate, which consists of a viral prohead and a DNA replicating intermediate (preinitiation complex). φX 174 mutants, ogr (rep), which restore the ability to propagate in the mutant rep hosts, were isolated. The ogr(rep) mutations confer amino acid substitutions in the viral coat protein, a constituent of the prohead, and the viral A protein, a constituent of the preinitiation complex. Four of the six coat protein substitutions are localized on or near the twofold axis of symmetry in the atomic structure of the mature virion.