BIOLOGICAL SIGNIFICANCE OF THE 7 AMINO-TERMINAL BASIC RESIDUES OF BROME MOSAIC-VIRUS COAT PROTEIN

Inoculation of six brome mosaic virus (BMV) RNA3 transcripts with defi ned deletions in the coat protein (CP) gene to three Chenopodium spp d emonstrated that synthesis of a functional, encapsidation-competent CP is required for the induction of local lesions. The BMV CP open readi ng frame contains two in-frame AUG codons separated by seven amino aci ds, resulting in the synthesis of two CPs (CP1 and CP2). To elucidate the biological significance of the N-terminal basic region of BMV CP, RNA3 variants capable of producing either CP1 or CP2 but not both were constructed. Infection phenotypes elicited on three Chenopodium spp b y each RNA3 variant revealed that amino-terminal residues 1 to 7 are r equired to establish chlorotic local lesions and systemic infection in Chenopodium quinoa. Deletion of this region has no effect on infectio n in barley plants but resulted in the induction of the hypersensitive response on the inoculated leaves of C. quinoa and blocked systemic s pread. Analysis of seven additional RNA3 variant transcripts, each hav ing a six-base deletion (two amino acids) in the sequence encoding the N-terminal seven residues, indicated that variants that share a commo n deletion of positively charged lysine rendered the CP encapsidation- incompetent and failed to establish infection. Taken together, these r esults suggest that residues 1 to 7 of the BMV CP play an important ro le in virus-host interactions and contribute differently to the virule nce phenotype in different host plants. (C) 1995 Academic Press, Inc.