Reovirus serotypes 1 (Lang strain) and 3 (Dearing strain) code for a hitherto unrecognized low-molecular-weight polypeptide of Mr ∼ 12,000. This polypeptide (p12) was synthesized in vitro in L-cell-free protein synthesizing systems programmed with either reovirus serotype 1 mRNA, reovirus serotype 3 mRNA, or with denatured reovirus genome double-stranded RNA, and in vivo in L-cell cultures infected with either reovirus serotype. The synthesis of p12 in vivo was insensitive to actinomycin D, and occurred at similar times after infection as the previously identified reovirus encoded λ, μ, and σ polypeptides. Pulse-chase experiments in vivo, and the relative kinetics of synthesis of p12 in vitro, indicate that it is a primary translation product. Fractionation of reovirus mRNAs by velocity sedimentation and translation of separated mRNAs in vitro suggests that p12 is coded for by the sl mRNA, which also codes for the previously recognized σ1 polypeptide. Synthesis of both p12 and a, in vitro in L-cell-free protein synthesizing systems programmed with denatured reovirus genome double-stranded RNA also suggests that these two polypeptides can be coded by the same mRNA species. The Mr ∼ 12,000 ∼ypeptide was not a detectable structural component of purified virions, and antiserum prepared against purified reovirions did not immunoprecipitate p12. It is proposed that the Mr ∼ 12,000 polypeptide encoded by the Sl genome segment be designated σ1bNS and that the polypeptide previously designated σ1 be renamed σ1a.
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