Specificity of in vitro reconstitution of bromegrass mosaic virus

Bromegrass mosaic virus (BMV) RNA was allowed to compete with yeast tRNA or alfalfa mosaic virus (AMV) RNA for in vitro encapsidation by BMV protein. Various proportions of 32P-labeled foreign RNAs were added to a reassembly mixture (BMV protein: BMV RNA, 4:1) and the reassembly products were observed by analytical and rate-zonal sedimentation, and the RNA contents of the nucleocapsids were examined by gel electrophoresis. Incubation of BMV protein with tRNA alone produced 56 S particles containing five or six tRNA molecules per particle, but with both tRNA and BMV RNA present very little of the tRNA was incorporated. AMV 12 S RNA led to 64 S particles containing one AMV RNA molecule: with both AMV and BMV RNAs present, the smallest BMV RNA outcompeted the AMV RNA about fourfold, even though the two RNAs have similar molecular weights and biological functions. Evidently BMV protein does to some extent specifically recognise its own RNA molecules.