STOICHIOMETRIC PACKAGING OF THE 3 GENOMIC SEGMENTS OF DOUBLE-STRANDED-RNA BACTERIOPHAGE-PHI-6

A model that explains the stoichiometric packaging of the chromosomes of Phi 6, a bacteriophage with a genome of three unique double-strande d RNA segments, is proposed and supported. Ordered switches in packagi ng specificity and RNA synthesis are determined by the amount of RNA w ithin the procapsid. The plus strand of segment S binds to one of seve ral sites on the outside of the empty procapsid. The RNA enters and th e procapsid expands so that the S sites are lost and M sites appear. P ackaging of segment M results in the loss of the M sites and the appea rance of the L sites. Packaging of L readies the particle for minus-st rand synthesis. If any of the segments is less than normal size, packa ging of that class of segments continues until the normal content of R NA for that segment is packaged and the binding sites then change.