STUDIES OF THE MAJOR REOVIRUS CORE PROTEIN SIGMA-2 - REVERSION OF THE ASSEMBLY-DEFECTIVE MUTANT TSC447 IS AN INTRAGENIC PROCESS AND INVOLVES BACK MUTATION OF ASP-383 TO ASN

The reovirus group C temperature-sensitive mutant tsC447, whose defect maps to the S2 gene, which encodes the major core protein sigma 2, fails to assemble core particles at the nonpermissive temperature. To identify other proteins that may interact,vith sigma 2 during assembly, we generated and examined 10 independent revertants of the mutant. To determine which gene(s) carried a compensatory suppressor mutation(s), we generated intertypic reassortants between wild-type reovirus serotype 1 Lang and each revertant and determined the temperature sensitivities of the reassortants by efficiency-of-plating assays. Results of the efficiency-of-plating analyses indicated that reversion of the tsC447 defect was an intragenic process in all revertants. To identify the region(s) of sigma 2 that had reverted, we determined the nucleotide sequences of the S2 genes. In all revertant sequences examined, the G at nucleotide position 1166 in tsC447 had reverted to the A present in the wild-type sequence. This reversion leads to the restoration of a wild-type asparagine (in place of a mutant aspartic acid) at amino acid 383 in the sigma 2 sequence. These results collectively indicate that the functional Lesion in tsC447 is Asp-383 and that this lesion cannot be corrected by alterations in other core proteins. These observations suggest that this region of sigma 2, which may be important in mediating assembly of the core particle, does not interact significantly with other reovirus proteins.