Coat protein gene duplication in a filamentous RNA virus of plants

Computer-assisted analysis revealed a striking sequence similarity between the putative 24-kDa protein (p24) encoded by open reading frame (ORF) 5 of beet yellows closterovirus and the coat protein of this virus encoded by the adjacent ORF6. Both of these proteins are closely related to the homologous proteins of another closterovirus, citrus tristeza virus. It is hypothesized that the genes for coat protein and its diverged tandem copy have evolved by duplication. Phylogenetic analysis using various methods for tree generation suggested that the duplication was already present in the genome of the common ancestor of the two closteroviruses. The genes for p24 and coat protein of beet yellows closterovirus were cloned, transcribed, and translated in vitro yielding products of the expected size. It was shown that p24 is translated starting from the first of the two alternative AUG codons located near the 5' terminus of ORF5. The presence of a single protein species in beet yellows closterovirus virions and the near identity of the amino acid composition of this protein with the composition of the ORF6 but not the ORF5 product indicated that p24 is not a major virion component. Most of the amino acids that are conserved in the coat proteins of filamentous viruses of plants are retained also in p24. These observations suggest that p24 may share some structural and functional features with the coat protein but probably fulfills a distinct function in virus reproduction.