POLYNUCLEOTIDE PHOSPHORYLASE OF ESCHERICHIA-COLI IS REQUIRED FOR THE ESTABLISHMENT OF BACTERIOPHAGE P4 IMMUNITY

Bacteriophage P4's superinfection immunity mechanism is unique among t hose of other known bacteriophages in several respects: (i) the P4 imm unity factor is not a protein but a short, stable RNA (CI RNA); (ii) i n the prophage the expression of the replication operon is prevented b y premature transcription termination rather than by repression of tra nscription initiation; (iii) transcription termination is controlled v ia RNA-RNA interactions between the CI RNA and two complementary targe t 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 int eractions, 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 freque ncy and delayed both the disappearance of the long transcripts that co ver the entire replication operon and the appearance of the CI RNA. By physical mapping and genetic analysis we show that bfl-1 is allelic t o pnp, which codes for polynucleotide phosphorylase, a 3'-to-5' exonuc leolytic enzyme, A previously isolated pnp null mutant (pnp-7) exhibit ed a phenotype similar to that of bfl-1, These results indicate that t he polynucleotide phosphorylase of E. coli is involved with the matura tion pathway of bacteriophage P4's RNA immunity factor.