MOLECULAR STUDIES ON BROMOVIRUS CAPSID PROTEIN .2. FUNCTIONAL-ANALYSIS OF THE AMINO-TERMINAL ARGININE-RICH MOTIF AND ITS ROLE IN ENCAPSIDATION, MOVEMENT, AND PATHOLOGY

The N-terminal region of the brome mosaic bromovirus (BMV) coat protei n (CP) contains an arginine-rich motif that is conserved among plant a nd nonplant viruses and implicated in binding the RNA during encapsida tion. To elucidate the functional significance of this conserved motif in the BMV CP, a series of deletions encompassing the arginine-rich m otif was introduced into a biologically active clone of BMV RNA3, and their effect on replication, encapsidation, and infection in plants wa s examined. Analysis of infection phenotypes elicited on Chenopodium q uinoa revealed the importance of the first 19 N-proximal amino acids o f BMV CP in encapsidation and pathogenicity. Inoculation of C. quinoa with three viable variants of BMV RNA3 lacking the first 11, 14, and 1 8 N-terminal amino acids of the CP resulted in the development of necr otic local lesions and restricted the spread of infection to inoculate d leaves. Progeny analysis from symptomatic leaves revealed that, in e ach case, virus accumulation was severely affected by the introduced m utations and each truncated CP differed in its ability to package geno mic RNA. In contrast to these observations in C. quinoa, none of the C P variants was able to establish either local or systemic infections i n barley plants. The intrinsic role played by the N-terminal arginine- rich motif of BMV CP in packaging viral RNAs and the interactions betw een the host and the truncated CPs in modulating symptom expression an d movement are discussed. (C) 1996 Academic Press Inc.