A SPONTANEOUS MUTATION IN THE MOVEMENT PROTEIN GENE OF BROME MOSAIC-VIRUS MODULATES SYMPTOM PHENOTYPE IN NICOTIANA-BENTHAMIANA

Brome mosaic virus (BMV) is a positive-strand RNA virus with a multipa rtite genome that causes symptomless infection in Nicotiana benthamian a. We have isolated and characterized a strain of BMV that produced un iform vein chlorosis in systemically infected N. benthamiana. Analysis of pseudorecombinants constructed by exchanging RNA 1 and 2 and RNA 3 components between wild-type (non-symptom-inducing) and vein chlorosi s-inducing strains of BMV indicated that the genetic determinant for t he induction of the chlorotic phenotype is located on RNA 3. Sequence analysis of progeny RNA 3 recovered from symptomatic N. benthamiana pl ants revealed that vein chlorosis is due to the single nucleotide tran sition (887)G-->(887)A, which changes the codon for Val-266 to Ile-266 in the movement protein gene. The mutation had no detectable effect o n the accumulation of virus in either inoculated or systemically infec ted leaves of N. benthamiana. The vein chlorosis phenotype is the mani festation of the substitution of Ile-266 for Val-266 in the movement p rotein gene, since additional alterations in this region (a silent mut ation, i.e., (887)GUU(889)-->GUC, and an alteration of valine to pheny lalanine, i.e., (887)GUU(889)-->(UUU889)-U-887) resulted in symptomles s infections on N. benthamiana, The modulation of the symptom phenotyp e by the substitution of Ile-266 for Val-266 is specific for N. bentha miana, since neither movement nor the symptom phenotype in barley plan ts was affected.