FIG. 36.

The role of the capsid-size-determining protein in the bacteriophage P2-P4 system is shown by a series of surface views of reconstructions. Bacteriophage P2 forms a T=7 particle of 600 Å in diameter (A) which packages its 33-kb genome efficiently. Bacteriophage P4 uses the proteins of P2 to form a 450-Å T=4 shell (B) which can package its smaller 11-kb genome but excludes the larger P2 one. The change in capsid size is controlled by the P4 gpSid protein. Reconstruction of a recombinant particle which resembles the 390-Å P4 procapsid (C) and the procapsid bearing a full complement of gpSid (D) shows that the size determination protein forms a 400-Å dodecahedral shell on the outside of the procapsid. Expansion after packaging brings the mature P4 to its final 450-Å diameter. Adapted from references 101 and 213 with the permission of the author and the publishers.