Fig. 1.

Schematic representation of the structural organization of a mature cystovirus virion. The membrane proteins that include the major envelope protein, P9 (90 residues in Φ6), an envelope protein, P10 (42 residues), and a minor membrane protein, P13 (72 residues) are associated with the outer lipid envelope. Exposed to the extracellular medium, is the receptor-binding spike protein, P3 (648 residues) that forms a complex with the fusogenic membrane protein, P6 (167 residues), is able to interact specifically with the type IV pili of the bacterial host in the case of Φ6 (and Φ2954). In some cystoviruses, e.g. in Φ8, P3 comprises of two separate proteins P3a and P3b. The outer protein capsid is formed by 200 trimers of the major outer capsid protein, P8 (149 residues) assembled on a T = 13 lattice. Not all of all the cystoviruses contain a P8 shell e.g. Φ8. The lytic protein P5 (220 residues) that is involved in the degradation of the peptidoglycan layer during internalization into the host cytoplasm is found in the space between the lipid and outer protein layers. The inner protein capsid is formed from asymmetric dimers of the inner capsid protein, P1 (769 residues) arranged on a T = 1 lattice. Associated with the P1 shell are three other proteins all encoded on the genomic L-segment: an RNA-directed RNA polymerase, P2 (664 residues), a hexameric packaging NTPase, P4 (332 residues) and an accessory protein, P7 (161 residues). The proteins P1, P2, P4 and P7 assemble to form the polymerase complex (PX). The three segments, large (L, 6.4 kbp), medium (M, 4.1 kbp) and small (S, 2.9 kbp) of the double-stranded RNA genome are enclosed within the P1 shell. The L-segment encodes for the P1, P2, P4 and P7 proteins, the M-segment encodes for the P3, P6, P10 and P13 proteins; the S-segment encodes P5, P8 and P9 proteins.