Inter-RNA interaction of phage phi29 pRNA to form a hexameric complex for viral DNA transportation

Ds-DNA viruses package their DNA into a preformed protein shell (procapsid) during maturation. Bacteriophage phi29 requires an RNA (pRNA) to package its genomic DNA into the procapsid. We report here that the pRNA upper and lower loops are involved in RNA/RNA interactions. Mutation in only one loop results in inactive pRNAs. However, mixing of two, three and six inactive mutant pRNAs restores DNA packaging activity as long as an interlocking hexameric ring can be predicted to form by base pairing of the mutated loops in separate RNA molecules. The stoichiometry of pRNA for the packaging of one viral DNA genome is six. Homogeneous pRNA purified from a single band in denaturing gels showed six bands when rerun in native gels. These results suggest that six pRNAs form a hexameric ring by the intermolecular interaction of two RNA loops to serve as part of the DNA transportation machinery.