Temperature-sensitive mutants of reovirus : V. Studies on the nature of the temperature-sensitive lesion of the group C mutant ts447

Studies were carried out to determine which of the polypeptides coded by the group C reovirus temperature-sensitive mutant ts447, a double-stranded RNA-negative mutant, is mutated. Evidence was obtained from two lines of experimentation:

1. (i) In cells infected with this mutant at nonpermissive temperatures the structures within which viral double-stranded RNA is normally synthesized (DSRSS) are not formed; further, when shifted from permissive to nonpermissive temperatures, the rate of DS RNA synthesis decays at approximately the same rate at which plus-stranded template is used up during double-stranded RNA synthesis in vitro. ts447 DSRSS that are formed at permissive temperatures can synthesize double-stranded RNA at nonpermissive temperatures in vitro, but they are more heat-labile than wt DSRSS. These results suggest that the ts447 polypeptide that is mutated is a structural component of DSRSS.

2. (ii) In contrast to wt reovirus, ts447 forms two types of empty, or top component, particles: normal ones that are indistinguishable from those of wt virus, and aberrant ones which possess a slightly lower buoyant density and a capsid that (1) is only about one-half as thick as that of wt top component particles, (2) appears to consist of only one rather than two shells, and (3) lacks the two innermost core polypeptides λ1 and σ2. Independent evidence suggests that core polypeptides are among the structural components of DSRSS; the two lines of evidence therefore converge to suggest that the mutation of ts447 is either in polypeptide λ1 or in polypeptide σ2.