THE BARLEY STRIPE MOSAIC-VIRUS GAMMA-B GENE ENCODES A MULTIFUNCTIONALCYSTEINE-RICH PROTEIN THAT AFFECTS PATHOGENESIS

Barley stripe mosaic virus contains seven genes, one of which specifie s a 17-kD cysteine-rich protein, gamma b, that is known to affect viru lence. To further characterize the role of gamma b in pathogenesis, we mutagenized sequences encoding amino acids within two clusters of cys teine and histidine residues in the cysteine-rich domain and a group o f basic amino acids located between the clusters and determined the ef fects of these mutations on the symptom phenotype in barley. Three sin gle amino acid substitutions in cluster 1 and two amino acid exchanges in the basic region caused bleached symptoms associated with pronounc ed elevations in accumulation of gamma b protein. In contrast, three s ingle amino acid substitutions in cluster 2 and a mutation in the basi c motif resulted in attenuated (''null'') symptoms typical of those pr oduced when the gamma b gene is deleted. Tissue infected with these '' null'' mutants accumulated slightly elevated amounts of the gamma b pr otein but significantly lower levels of coat protein and the putative movement protein beta b. Genetic complementation tests revealed that c luster 1 mutations are dominant over the wild-type gamma b gene, where as those in cluster 2 are recessive. These results highlight the pivot al role of gamma b in pathogenesis and suggest that the two cysteine-r ich clusters are functionally distinct and that they affect different aspects of disease development.