EFFECTS OF MUTATIONS IN THE BEET WESTERN YELLOWS VIRUS READTHROUGH PROTEIN ON ITS EXPRESSION AND PACKAGING AND ON VIRUS ACCUMULATION, SYMPTOMS, AND APHID TRANSMISSION

Virions of beet western yellows luteovirus contain a major capsid prot ein (P22.5) and a minor readthrough protein (P74), produced by transla tional readthrough of the major capsid protein sequence into the neigh boring open reading frame, which encodes the readthrough domain (RTD). The RTD contains determinants required for efficient virus accumulati on in agroinfected plants and for aphid transmission. The C-terminal h alves of the RTD are not well conserved among luteoviruses but the N-t erminal halves contain many conserved sequence motifs, including a pro line-rich sequence separating the rest of the RTD from the sequence co rresponding to the major coat protein. To map different biological fun ctions to these regions, short in-frame deletions were introduced at d ifferent sites in the RTD and the mutant genomes were transmitted to p rotoplasts as transcripts and to Nicotiana clevelandii by agroinfectio n. Deletions in the nonconserved portion of the RTD did not block aphi d transmission but had a moderate inhibitory effect on virus accumulat ion in plants and abolished symptoms. Deletion of the proline tract an d the junction between the conserved and nonconserved regions inhibite d readthrough protein accumulation in protoplasts by at least 10-fold. The mutants accumulated small amounts of virus in plants, did not ind uce symptoms, and were nontransmissible by aphids using agroinfected p lants, extracts of infected protoplasts, or purified virus as a source of inoculum. Other deletions in the conserved portion of the RTD did not markedly diminish readthrough protein accumulation but abolished i ts incorporation into virions. These mutants accumulated to low levels in agroinfected plants and elicited symptoms, but could not be aphid- transmitted. A preliminary map has been produced mapping these functio ns to different parts of the RTD. (C) 1997 Academic Press.