Coding density of the turnip yellow mosaic virus genome: roles of the overlapping coat protein and p206-readthrough coding regions.

More than one-third of the turnip yellow mosaic virus (TYMV) genome simultaneously encodes two ORFs. We have investigated the functions of the overlapping coat protein ORF and readthrough domain of ORF-206 in the 3' region of the genome. TYMC-206 RNA, in which a second stop codon has been positioned to prevent ORF-206 readthrough, induced infections in protoplasts and plants that were indistinguishable from wild type. ORF-206 readthrough is thus nonessential. Nevertheless, TYMV-221 RNA, in which the ORF-206 stop codon was replaced with a tyrosine codon to force readthrough, was infectious to protoplasts, suggesting that a role for ORF-206 readthrough under certain conditions is possible. TYMV RNA variants that produce truncated or no coat protein were used to show that the coat protein is dispensable for local movement but necessary for systemic spread of virus in plants. Studies in protoplasts showed that (-) RNA levels are normal in the absence of coat protein, but (+) strand levels are decreased about 10-fold relative to wild-type infections. A mutant with a short C-terminal coat protein extension that formed virions less stable than normal demonstrated the protective role of capsids toward genomic RNA. The evolutionary implications of the dense information content of the TYMV genome are discussed.