In vivo studies of genomic packaging in the dsRNA bacteriophage Phi8

Phi8 is a bacteriophage containing a genome of three segments of double-stranded RNA inside a polyhedral capsid enveloped in a lipid-containing membrane. Plus strand RNA binds and is packaged by empty procapsids. Whereas Phi6, another member of the Cystoviridae, shows high stringency, serial dependence and precision in its genomic packaging in vitro and in vivo, Phi8 packaging is more flexible. Unique sequences (pac) near the 5' ends of plus strands are necessary and sufficient for Phi6 genomic packaging and the RNA binding sites are located on P1, the major structural protein of the procapsid.

In this paper the boundaries of the Phi8 pac sequences have been explored by testing the in vivo packaging efficacy of transcripts containing deletions or changes in the RNA sequences. The pac sequences have been localized to the 5' untranslated regions of the viral transcripts. Major changes in the pac sequences are either tolerated or ameliorated by suppressor mutations in the RNA sequence. Changes in the genomic packaging program can be established as a result of mutations in P1, the major structural protein of the procapsid and the determinant of RNA binding specificity.

Although Phi8 is distantly related to bacteriophage Phi6, and does not show sequence similarity, it has a similar genomic packaging program. This program, however, is less stringent than that of Phi6.