Encapsidation of the Segmented Double-Stranded RNA Genome of Bacteriophage φ6

Abstract

Bacteriophage φ6 has a segmented double-stranded RNA genome that is incorporated into a preformed capsid during viral assembly. The three viral genomic segments are packaged as single-stranded precursors, which are later replicated into the mature double-stranded genome inside the capsid by the viral polymerase. The packaging efficiency of φ6 is high; virtually all particles released from φ6-infected cells are infectious and carry one copy of each of the genome segments. This feature makes φ6 an appropriate model to analyze the principles of packaging of a multi-segmented genome. In vitro analyses have revealed that the packaging of φ6 involves sequential uptake of the three genome segments. The initiation of RNA replication is a checkpoint for correct genome packaging and is dependent on a specific interaction between the viral RNA and the preassembled polymerase complex, the procapsid. During the maturation the polymerase complex undergoes structural changes which lead to the expansion of the particle. The packaging NTPase of φ6 is a ring-like hexamer that is located at each of the five-fold vertices in the procapsid. However, it appears that only one vertex is adequate to carry out efficient packaging, while the others are required when the newly synthesized message RNAs exits from the capsid. φ6 packaging and replication shares features with bacterial double-stranded DNA viruses as well as with eukaryotic double-stranded RNA viruses.