Precise packaging of the three genomic segments of the double-stranded-RNAbacteriophage phi 6

Bacteriophage phi 6 has a genome of three segments of double-stranded RNA. Each virus particle contains one each of the three segments. Packaging is e ffected by the acquisition, in a serially dependent manner, of the plus str ands of the genomic segments into empty procapsids. The empty procapsids ar e compressed in shape and expand during packaging The packaging program inv olves discrete steps that are determined by the amount of RNA inside the pr ocapsid. The steps involve the exposure and concealment of binding sites on the outer surface of the procapsid for the plus strands of the three genom ic segments. The plus strand of segment S can be packaged alone, while pack aging of the plus strand of segment M depends upon prior packaging of S. Pa ckaging of the plus strand of L depends upon the prior packaging of M. Minu s-strand synthesis begins when the particle has a full complement of plus s trands. Plus-strand synthesis commences upon the completion of minus-strand synthesis. All of the reactions of packaging, minus-strand synthesis, and plus-strand synthesis can be accomplished in vitro with isolated procapsids . Live-virus constructions that are in accord with the model have been prep ared. Mutant virus with changes in the packaging program have been isolated and analyzed.