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Mutation of the Tobacco Mosaic Tobamovirus 126-and 183-kDa Proteins: Effects on Phloem-Dependent Virus Accumulation and Synthesis of Viral Proteins

The masked and U1 strains of tobacco mosaic tobamovirus differ in symptom phenotype and in phloem-dependent accumulation in tobacco. The symptom phenotype is determined by eight amino acids in the 126- and 183-kDa proteins that differ between the two strains. In this study, slow phloem-dependent accumulation of the masked strain was shown to be determined by these same eight amino acids, but some symptomatically severe mutants altered at specific positions within the eight amino acids were inefficient in phloem-dependent accumulation. Therefore, the appearance of severe symptoms does not require rapid phloem-dependent accumulation. There was no consistent relationship between the accumulation of virus coat protein, movement protein, or 126- and 183-kDa protein in inoculated protoplasts and the efficiency of phloem-dependent accumulation in stem tissue. Therefore, the difference in phloem-dependent accumulation between the masked strain, its mutants, and the U1 strain in most instances resulted from the functional competence of the 126- and/or 183-kDa proteins or a host response to their change and not from their quantities or the quantities or functional competence of the movement proteins or coat proteins.