The immI region of Salmonella phage P22. 1. Characterisation of the mutants P22 cir4-1, P22 cir5-1 and P22 cir6-1.

Salmonella phage mutants P22 cir4-1, P22 cir5-1 and P22 cir6-1 at 37 degrees C form more or less clear plaques. The mutants complement each other and clear mutations of the immC region (Prell et al. 1983). The mutants exhibit a strongly reduced frequency of lysogenisation, but form stable prophages. The low frequency of lysogenisation of P22 cir5-1 and of P22 cir6-1 is suppressed by an additional ant- mutation. Similarly, about 50% of turbid plaque revertants of both mutants carry ant- suppressor mutations. This suggests interference by the cir5-1 and cir6-1 mutations with the expression of gene ant. In contrast, the cir4-1 mutation seems not to interfere with ant expression, the latent periods of P22 cir4-1 and P22 cir5-1 are reduced and extended, respectively. The geneticly related Salmonella phage L carries a gene able to complement P22 cir4-1.