Identification of a new polypeptide coded by reovirus gene S1

The reovirus S1 gene has recently been shown potentially to encode two polypeptides (from two overlapping reading frames) having predicted molecular weights of 49,071 and 16,143 (Nagata et al., Nucleic Acids Res. 12:8699-8710, 1984; Bassel-Duby et al., Nature [London], in press). The larger polypeptide is reovirus protein sigma 1, but synthesis of the smaller polypeptide has not been described to date. A truncated clone of the S1 gene in which the first ATG is deleted was expressed in an in vitro protein synthesis system to yield a approximately 13-kilodalton polypeptide, as determined from migration on sodium dodecyl sulfate-polyacrylamide gels. A polypeptide with a similar migration pattern on sodium dodecyl sulfate-polyacrylamide gels was present in reovirus-infected cells and absent from mock-infected cells. Comparative tryptic peptide analysis of the 13-kilodalton polypeptides produced in vivo and in vitro showed them to be identical. Thus, the s1 mRNA of reovirus type 3 is apparently bicistronic, and we suggest that the approximately 13-kilodalton polypeptide be called sigma s (standing for sigma small).