Polynucleotide phosphorylase of Escherichia coli is required for the establishment of bacteriophage P4 immunity

Bacteriophage P4's superinfection immunity mechanism is unique among those of other known bacteriophages in several respects: (i) the P4 immunity factor is not a protein but a short, stable RNA (CI RNA); (ii) in the prophage the expression of the replication operon is prevented by premature transcription termination rather than by repression of transcription initiation; (iii) transcription termination is controlled via RNA-RNA interactions between the CI RNA and two complementary target sequences on the nascent transcript; and (iv) the CI RNA is produced by processing of the same transcript it controls. It was thought that several host-encoded factors may participate in the molecular events required for P4 immunity expression, i.e., RNA processing, RNA-RNA interactions, and transcription termination. To identify such factors we searched for Escherichia coli mutations that affect P4 lysogenization. One such mutation, bfl-1, severely reduced P4's lysogenization frequency and delayed both the disappearance of the long transcripts that cover the entire replication operon and the appearance of the CI RNA. By physical mapping and genetic analysis we show that bfl-1 is allelic to pnp, which codes for polynucleotide phosphorylase, a 3'-to-5' exonucleolytic enzyme. A previously isolated pup null mutant (pnp-7) exhibited a phenotype similar to that of bfl-1. These results indicate that the polynucleotide phosphorylase of E. coli is involved with the maturation pathway of bacteriophage P4's RNA immunity factor.