Three complementation groups in bacteriophage R17.

Complementation between amber mutants of R17 reveals the existence of three phage cistrons, designated A, B and C. Cistron A specifies a protein necessary for the synthesis of infective phage particles; cistron B specifies phage coat protein; and cistron C codes for phage-specific RNA synthetase. One of the mutants of group B is a polar coat-protein mutant, unable to synthesize either phage coat protein or RNA synthetase. Two models for polarity based on the interaction between ribosomes and the phage message during protein synthesis are discussed.