Identification of a coat protein binding site on southern bean mosaic virus RNA.

The cowpea strain of Southern bean mosaic virus (SBMV-C), a T = 3 icosahedral RNA virus, was dissociated to yield a ribonucleoprotein complex (RNPC) composed of the viral RNA and coat protein subunits. To determine if the coat protein subunits were bound to a specific site on the viral RNA, the RNPC was treated with ribonuclease, and the remaining coat protein--RNA complexes were recovered by filter binding. A single species of RNA was isolated by this procedure and further characterized by sequencing. The RNA was mapped to nucleotides 1410-1436 of the SBMV-C genome. This region of the viral RNA was predicted to fold into a hairpin with a 4-base loop and a duplex stem of 24 nucleotides. The stability and specificity of the coat protein-RNA complex isolated from dissociated virus suggest a possible role for this interaction in the selective encapsidation of the viral RNA.