Mechanism of action of Salmonella phage P22 antirepressor

On the basis of genetic evidence, Botstein et al. (1975) and Levine et al. (1975) proposed that phage P22 specifies an antirepressor which interferes with the synthesis or activity of the P22 c 2 repressor. In this paper, we show that P22 anti-repressor interferes with phage λ cI repressor (which is analogous in function to P22 c 2 repressor), since superinfection of a λ lysogen with P22 results in dis-appearance of λ repressor activity. Antirepressor interferes directly with repressor activity rather than indirectly with repressor synthesis, since antirepressor in crude P22-infected cell extracts inhibits λ repressor activity in vitro. Under our assay conditions, antirepressor action requires pre-incubation of antirepressor with λ repressor before addition of λ DNA, suggesting that antirepressor works by interacting with repressor rather than by interacting with the DNA operators. We can rule out the hypothesis that antirepressor inactivates repressor by covalently modifying or cleaving it, since repressor that has been inactivated by antirepressor can be recovered in active form by treatment with sodium dodecyl sulfate and urea, or by treatment with guanidine. Taken together, these results suggest that antirepressor inactivates repressor by binding non-covalently to it.