A NOVEL OPEN READING FRAME IN TOBACCO MOSAIC-VIRUS GENOME CODING FOR A PUTATIVE SMALL, POSITIVELY CHARGED PROTEIN

From sequence comparisons between the tobamovirus genomes an open read ing frame (ORF-X) potentially encoding a small, positively charged pro tein (33- to 45-amino-acids long) was found to overlap the immediate 3 ' and 5' sides of the transport protein gene and coat protein gene, re spectively. In vitro translation of the monocistronic artificial trans cripts generated with T7 RNA polymerase yielded a protein of M(r) 4000 (p4) and an unexpected trypsin-sensitive complex of M(r) 54000 that w as resistant to reduction with 2-mercaptoethanol but could be dissocia ted by 8 M urea. Assembly of this complex was inhibited completely by site-directed mutagenesis within a conserved, positively charged 5-ami no-acid long segment of the ORF-X protein. After centrifugation in low salt buffer the 54-kDa complex remained mostly associated with riboso mes. Apparently this complex represents a specific aggregate of the p4 product of ORF-X with a protein of approximate M(r) 50000 that is a c omponent of the translation apparatus.