CONTROL OF TRANSCRIPTION TERMINATION BY AN RNA FACTOR IN BACTERIOPHAGE P4 IMMUNITY - IDENTIFICATION OF THE TARGET SITES

Prophage P4 immunity is elicited by a short, 69-nucleotide RNA (CI RNA ) coded for within the untranslated leader region of the same operon i t controls. CI RNA causes termination of transcription that starts at the promoter P-LE and prevents the expression of the distal part of th e operon that codes for P4 replication functions (alpha operon), In th is work, we identify two sequences in the untranslated leader region o f the alpha operon, seqA and seqC, that are the targets of the P4 immu nity factor, seqA and seqC exhibit complementarity to a sequence inter nal to the CI RNA (seqB). Mutations in either seqA or seqC that alter its complementarity to seqB abolished or reduced P4 lysogenization pro ficiency and delayed the shutoff of the long transcripts originating f rom P-LE that cover the entire operon. Both seqA and seqC single mutan ts were still sensitive to P4 prophage immunity, whereas P4 seqA seqC double mutants showed a virulent phenotype, Thus, both functional site s are necessary to establish immunity upon infection, whereas a single site appears to be sufficient to prevent lytic gene expression when i mmunity is established. A mutation in seqB that restored complementari ty to both seqA and seqC mutations also restored premature termination of P-LE transcripts, thus suggesting an important role for RNA-RNA in teractions between seqB and seqA or seqC in P4 immunity.