A comparative study on the in vitro translation products of individual RNAs from two-, three-, and four-component strains of barley stripe mosaic virus.

Passaging through plants of the three-component barley stripe mosaic virus (BSMV) strain Norwich (Norwich III) yielded a two-component isolate (Norwich II). A study was made of the sets of polypeptides translated in a rabbit reticulocyte and wheat embryo cell-free systems from individual RNAs of (1) the natural three-component strain Norwich III, (2) a two-component isolate derived from the former (Norwich II), and (3) an isolate intermediate between the three- and two-component ones, Norwich;. Translation of RNA 1 from all three variants of the Norwich strain in vitro yields a single major product with a molecular weight (Mr) of 120,000 (p120). RNA 2 from Norwich III in vitro produces two polypeptides: the viral coat protein (p23) and, in certain ionic conditions, a polypeptide of 25,000 M(r) (p25). RNA 3 from Norwich III is translated into a protein with Mr of 75,000 (p75). Conversion of Norwich III into Norwich II is accompanied by the changes in the coding specificity of RNA 2; beside the coat protein and p25, a protein of 85,000 M(r) (p85) is formed upon its translation-a feature characteristic of RNA 2 of the naturally occurring bipartite BSMV strains (Russian, type). With the Norwich(i) variety, which is marked by a significantly reduced portion of RNA 3 in the total virion RNA preparation, RNA 2 in addition to the coat protein, p25, and p85 directs the synthesis of another product with M(r) of about 78,000. Thus, in conversion of the three-component BSMV into a two-component one, the loss of RNA 3 is concomitant with the actuation in RNA 2 of a sequence coding for p85. RNAs 1-3 of the quadripartite Argentina Mild (AM) BSMV strain code in vitro for the same polypeptides as RNAs 1-3 of Norwich III. AM RNA 4 is translated as a monocistronic template into a polypeptide with Mr of 55,000 (p55). Amino acid sequences of p85, p75, and p55 are shown to overlap among these polypeptides but not to appreciably overlap with p120 sequences. Data presented here allowed a tentative structure to be proposed for genome of two-, three-, and four-component BSMV strains.