Structure of the bacteriophage phi6 nucleocapsid suggests a mechanism for sequential RNA packaging

Bacteriophage phi6 is an enveloped dsRNA virus with a segmented genome. Phi6 specifically packages one copy of each of its three genome segments into a preassembled polymerase complex. This leads to expansion of the polymerase complex, minus and plus strand RNA synthesis, and assembly of the nucleocapsid. The phi6 in vitro assembly and packaging system is a valuable model for dsRNA virus replication. The structure of the nucleocapsid at 7.5 A resolution presented here reveals the secondary structure of the two major capsid proteins. Asymmetric P1 dimers organize as an inner T = 1 shell, and P8 trimers organize as an outer T = 13 laevo shell. The organization of the P1 molecules in the unexpanded and expanded polymerase complex suggests that the expansion is accomplished by rigid body movements of the P1 monomers. This leads to exposure of new potential RNA binding surfaces to control the sequential packaging of the genome segments.