The immI region of Salmonella phage P22. 2. The influence of the cir mutations on the regulation of antirepressor synthesis.

The effects were studied of three clear plaque mutations of phage P22 (cir4-1, cir5-1 and cir6-1) on antirepressor synthesis. The mutant site cir4-1 has no influence on the expression of gene ant. The cir5-1 and cir6-1 mutations prevent the repression of early ant synthesis shortly after infection: P22 cir5-1 exhibits a strong ant-overproduction because it renders the cir5 repressor protein defective for turning off early ant expression (Harvey et al. 1981). P22 cir6-1 exhibits only a low level of constitutive ant synthesis insensitive to cir5-directed repression. Complementation experiments between P22 wild type or mutant in the cir5 or cir6 sites reveal the following phenotypes of the cir6-1 mutation: (1) The cir6-1 site behaves as a weak promotor site insensitive to cir5-directed repression of ant synthesis. (2) In the P22 cir5-1 cir6-1 double recombinant the cir6-1 site is cis dominant preventing ant overproduction as observed in simple P22 cir5-1 infections. (3) Some of the deficient phenotypes of P22 cir6-1 can be complemented by co-infecting P22 mutants carrying a cir6+ allele. These results are explained by the following model: The active cir5 repressor protein is a dimer (or oligomer) and the cir5-1 and cir6-1 mutations map in different domains influencing the repressing as well as the dimer forming activities of this protein. Which particular cir6-1 phenotype is observed depends on the experimental conditions employed, and on the promotor site (either pANT or cir6-1) from which ant expression procedes.(ABSTRACT TRUNCATED AT 250 WORDS)