Immunity specificity determinants in the P4-like retronphage φR73

Retronphage φR73 exhibits extensive sequence homology to the satellite bacteriophage P4. Bacteriophage P4 superinfection immunity is elicited by a small RNA (CI RNA) that causes premature transcription termination within the operon coding for the P4 replication functions. This control is exerted via interaction of the CI RNA with two complementary target sites on the untranslated leader RNA of the replication operon. We found that φR73 is endowed with a similar immunity system but is heteroimmune to P4. The heteroimmunity is due to six base differences in the CI RNA and to compensatory base substitutions in the target sequences. The sequence differences in the CI RNA are all located in single-stranded regions, which appear to play a predominant role in the interaction with the target sites. Analysis of phage carrying a hybrid immunity system indicates that, although two target sequences are required for the establishment of lysogeny, a single site is sufficient to make a phage sensitive to the prophage immunity.