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.
Kim
K J
KJ
Department of Microbiology, University of Iowa, Iowa City, IA 52242, USA.
Sunshine
M G
MG
Lindqvist
B H
BH
Six
E W
EW
D017901:Bacteriophage P2 / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D002213:Capsid / Q000235:genetics* / Q000378:metabolism*; D036022:Capsid Proteins; D003090:Coliphages / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism; D004926:Escherichia coli / Q000821:virology; D005814:Genes, Viral; D008242:Lysogeny; D008969:Molecular Sequence Data; D009154:Mutation; D019590:Nucleocapsid Proteins / Q000235:genetics*; D010957:Plasmids / Q000235:genetics; D017422:Sequence Analysis, DNA; D013489:Suppression, Genetic; D013696:Temperature
D036022:Capsid Proteins; D019590:Nucleocapsid Proteins; C095129:Sid protein, Bacteriophage P4