Characterization of deletion and frameshift mutants of satellite tobacco mosaic virus

A series of frameshift and deletion mutations was created in the genome of satellite tobacco mosaic virus (STMV) by modifying full-length cDNA clones of the type strain, from which biologically active transcripts could be synthesized in vitro. Deletions and frameshift mutations in the 5' open reading frame had no effect, compared to wild-type STMV, on RNA accumulation, systemic movement, or the symptoms induced by STMV in Nicotiana tabacum co-inoculated with tobacco mild green mosaic tobamovirus (TMGMV). This implies that the protein encoded by this reading frame is not necessary for biological activity. Deletions and frameshift mutations in the coat protein open reading frame resulted in decreased accumulation of STMV RNA in N. tabacum, although these mutants were still capable of systemic movement, presumably in a nonencapsidated or free RNA form. Furthermore, the mild symptoms induced in tobacco by co-inoculations of wild-type STMV/TMGMV or infection with TMGMV alone were altered to severe systemic necrosis when plants were co-inoculated with these STMV coat protein mutants and TMGMV. Mutants within the 3' untranslated region were much less able to accumulate in TMGMV-infected plants than was wild-type STMV, and under some growth conditions did not accumulate to detectable levels.