Translational effects and sequence comparisons of the three serotypes of the reovirus S4 gene

Reovirus S4 RNA codes for the dsRNA-binding polypeptide sigma 3, a major virion outer capsid component that also has translational effects in both infected and transfected mammalian cells. To compare the composition and properties of the three different serotypes of sigma 3, a DNA copy of the type 2 gene was cloned and sequenced. The total lengths (1196) and the sequences of leader (33 nucleotides) and trailer (66 nucleotides) regions are highly conserved among the three S4 serotypes. The type 1 and 3 S4 genes are highly related (77 mismatches). However, the type 2 gene contains many mismatches relative to the type 1 and 3 genes (260 and 270 positions, respectively). Most of the mismatches are third position changes, resulting in sigma 3 polypeptides that are 90% or more identical. Transient expression vectors, constructed by replacing the chloramphenicol acetyltransferase (CAT) gene in pRSVCAT with S4 DNA, were used to test the effects of polypeptide sigma 3 on CAT expression in cotransfected COS cells. Transfection with the correctly oriented DNAs resulted in synthesis of the corresponding sigma 3 polypeptides which enhanced CAT expression. The type 2 and type 3 S4 genes were considerably more stimulatory than type 1 when compared to CAT DNA alone. However, with all three serotypes the CAT activity was significantly higher in cells cotransfected with S4 DNA in the correct orientation as compared to the reverse arrangement.