Subunit folds and maturation pathway of a dsRNA virus capsid.

The cystovirus ϕ6 shares several distinct features with other double-stranded RNA (dsRNA) viruses, including the human pathogen, rotavirus: segmented genomes, nonequivalent packing of 120 subunits in its icosahedral capsid, and capsids as compartments for transcription and replication. ϕ6 assembles as a dodecahedral procapsid that undergoes major conformational changes as it matures into the spherical capsid. We determined the crystal structure of the capsid protein, P1, revealing a flattened trapezoid subunit with an α-helical fold.

The template specificity of bacteriophage Phi6 RNA polymerase.

Bacteriophage Φ6 contains three double-stranded RNA (dsRNA) genomic segments, L, M, and S. The RNA is located inside a core particle composed of multiple copies of a major structural protein, an RNA-dependent RNA polymerase, a hexameric NTPase, and an auxiliary protein. The virion RNA polymerase in the core particle transcribes segments M and S in vitro.

Packaging accessory protein P7 and polymerase P2 have mutually occluding binding sites inside the bacteriophage 6 procapsid.

Bacteriophage 6 is a double-stranded RNA (dsRNA) virus whose genome is packaged sequentially as three single-stranded RNA (ssRNA) segments into an icosahedral procapsid which serves as a compartment for genome replication and transcription. The procapsid shell consists of 60 copies each of P1(A) and P1(B), two nonequivalent conformers of the P1 protein. Hexamers of the packaging ATPase P4 are mounted over the 5-fold vertices, and monomers of the RNA-dependent RNA polymerase (P2) attach to the inner surface, near the 3-fold axes.

Packaging in dsRNA viruses.

Several families of viruses have segmented genomes with 3-12 chromosomes. They are capable of packaging these segments in a precise manner so that each virus particle contains one each of the genomic segments. The Cystoviridae are a family of bacteriophages that contain three genomic segments of dsRNA.

Stepwise expansion of the bacteriophage ϕ6 procapsid: possible packaging intermediates.

The initial assembly product of bacteriophage ϕ6, the procapsid, undergoes major structural transformation during the sequential packaging of its three segments of single-stranded RNA. The procapsid, a compact icosahedrally symmetric particle with deeply recessed vertices, expands to the spherical mature capsid, increasing the volume available to accommodate the genome by 2.5-fold.