Biosynthesis of reovirus-specified polypeptides The s1 mRNA synthesized in vivo is structurally and functionally indistinguishable from in vitro-synthesized s1 mRNA and encodes two polypeptides, σ_1a and σ_1bNS

The structural and functional properties of the reovirus serotype 1 (Lang strain) sl mRNA were examined. Reovirus s-class mRNAs, synthesized either in vivo within infected mouse L cells or in vitro by chymotrypsin-derived cores of purified virions, were purified by filter-hybridization using cDNA clones of the S-class genome segments. S1 cDNA-selected mRNA encoded the synthesis of the M_r ∼ 12,000 nonstructural polypeptide designated σ_1bNS in addition to the well-established structural polypeptide σ₁ now designated σ_1a. The coding properties of in vivo- and in vitro-synthesized s1 mRNA were equivalent: both encoded σ_1a and σ_1bNS. Primer extension analysis of s1 mRNA revealed a single major 5′ terminus for both in vivo- and in vitro-synthesized s1 mRNA. These results suggest that there is a single transcript of the reovirus S1 genome segment which is functionally dicistronic, and likely encodes both σ_1a and σ_1bNS.