The activity of the CIII regulator of lambdoid bacteriophages resides within a 24-amino acid protein domain

The CIII protein of lambdoid bacteriophages promotes lysogeny by stabilizing the phage-encoded CII protein, a transcriptional activator of the repressor and integrase genes. We have isolated a set of missense mutations in the cIII gene of phage λ and of phage HK022 that yield inactive CIII proteins. All the mutations are located in the relatively conserved central region of the protein. A comparative analysis of the CIII protein sequence in λ, HK022, and the lambdoid bacteriophage P22 leads us to suggest that this central region assumes an amphipathic α-helical structure. This part of the λ cIII gene was cloned within a fragment of the lacZ gene (the λ-complementing fragment). The resulting fusion protein displays CIII activity. Mutations that yield a nonfunctional fusion protein map within its CIII moiety. These results indicate that the central portion of the CIII protein is both necessary and sufficient for CIII activity.