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 Mr ∼ 12,000 nonstructural polypeptide designated σ1bNS in addition to the well-established structural polypeptide σ1 now designated σ1a. The coding properties of in vivo- and in vitro-synthesized s1 mRNA were equivalent: both encoded σ1a and σ1bNS. 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 σ1a and σ1bNS.
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