Capsid Size Determination in the P2–P4 Bacteriophage System: Suppression of sir Mutations in P2's Capsid Gene N by Supersid Mutations in P4's External Scaffold Gene sid

The sid gene of the P2-dependent phage P4 provides an external scaffold so P2 N gene encoded protomers assemble as T = 4 capsids rather than as P2's T = 7 capsids. Mutations (sir) in the middle of N interfere with Sid's function. We describe a new P4 mutant class, nms (“supersid”) mutations, which direct also P2 sir to provide small capsids. Three different nms mutations were located near the sid end, commingled with sid− mutations. Suppression of sir by nms is not allele-specific. Our results favor this interpretation of capsid size control: (i) sir mutations reduce pN protomer flexibility and thereby interfere with the generation of T = 4 compatible hexons; (ii) the C-termini of Sid molecules link up when forming the scaffold; nms mutations strengthen these Sid–Sid contacts and thus allow the scaffold to force even sir-type protomers to form T = 4 compatible hexons. Some related findings concern suppression of N ts mutations by P4.