READTHROUGH PROTEIN ASSOCIATED WITH VIRIONS OF BARLEY YELLOW DWARF LUTEOVIRUS AND ITS POTENTIAL ROLE IN REGULATING THE EFFICIENCY OF APHID TRANSMISSION

Purified particles of barley yellow dwarf luteovirus (BYDV) contain a major 22-kDa protein and a minor protein of approximately 58 kDa. The 22-kDa capsid protein is encoded by open reading frame (ORF) 3. ORF 5 is immediately downstream and in frame with ORF 3 and a 72-kDa protein can be translated via a readthrough suppression of the ORF 3 terminat ion codon. Antibodies were produced against two Escherichia coli expre ssed polypeptides that represent the amino- and carboxyl-terminal halv es of a putative 50-kDa protein encoded by ORF 5. Immunological analys es indicated that the 58-kDa protein associated with purified virions contained sequences encoded by ORF 3 and ORF 5. The carboxyl terminal portion of the full-length (72 kDa) readthrough protein was absent fro m the 58-kDa protein. The full-length readthrough protein was detected in infected oat protoplasts and plant tissue, but was not associated with virus particles purified from plants. The carboxyl-terminal porti on of the 72-kDa readthrough protein was not required for aphid transm ission; however, virus was transmitted more efficiently from protoplas t extracts containing virions and soluble 72-kDa readthrough protein t han from mock-inoculated protoplast extracts to which plant purified v irus was added. The full-length readthrough protein, although not requ ired for transmission, may increase the transmission efficiency of BYD V by aphids. (C) 1995 Academic Press, Inc.