IMMUNITY SPECIFICITY DETERMINANTS IN THE P4-LIKE RETRONPHAGE PHI-R73

Retronphage phi R73 exhibits extensive sequence homology to the satell ite bacteriophage P4. Bacteriophage P4 superinfection immunity is elic ited by a small RNA (CI RNA) that causes premature transcription termi nation within the operon coding for the P4 replication functions. This control is exerted via interaction of the CI RNA with two complementa ry target sites on the untranslated leader RNA of the replication oper on. We found that phi R73 is endowed with a similar immunity system bu t is heteroimmune to P4. The heteroimmunity is due to six base differe nces in the CI RNA and to compensatory base substitutions in the targe t sequences. The sequence differences in the CI RNA are all located in single-stranded regions, which appear to play a predominant role in t he interaction with the target sites. Analysis of phage carrying a hyb rid immunity system indicates that, although two target sequences are required for the establishment of lysogeny, a single site is sufficien t to make a phage sensitive to the prophage immunity. (C) 1996 Academi c Press, Inc.