Translation of soybean mosaic virus RNA in vitro: evidence of protein processing

The genomic RNA of soybean mosaic virus (SMV), a member of the potyvirus group of plant viruses, was translated in both the wheat germ and reticulocyte cell free systems to identify some viral encoded proteins and as an approach to determining the translational strategy of the virus. The RNA was translated into the same specific set of 10 to 12 polypeptides in both in vitro systems. Immunological tests and peptide analyses indicate that six translation products are related to SMV coat protein, and one of these comigrated with coat protein during electrophoresis. Two other antigenically distinct classes of polypeptides were identified by their specific immunoprecipitation with antibody against the SMV cytoplasmic inclusion body protein or tobacco etch virus nuclear inclusion body protein. To determine if any products of the in vitro translation reactions resulted from proteolytic processing of a precursor molecule, translation reactions were carried out with amino terminal label N-formyl[35S]methionyl-tRNA(Met)i (f-Met), or with [35S]methionine, and the resultant products were compared. The putative SMV coat protein and a translation product related to the nuclear inclusion body protein were not labeled with f-Met indicating that they were generated by proteolytic processing at their amino termini. Consistent with this finding is the accumulation of new polypeptides of greater apparent molecular weight when amino acid analogs were present during translation.