A RHO-DEPENDENT TRANSCRIPTION TERMINATION SITE REGULATED BY BACTERIOPHAGE P4 RNA IMMUNITY FACTOR

The genes required for replication of the temperate bacteriophage P4, which are coded by the phage left operon, are expressed from a constit utive promoter (P-LE) In the lysogenic state, repression of the P4 rep lication genes is achieved by premature transcription termination. The leader region of the left operon encodes all the genetic determinants required for prophage immunity, namely: (i) the P4 immunity factor, a short, stable RNA (Ct RNA) that is generated by processing of the lea der transcript; (ii) two specific target sequences that exhibit comple mentarity with the CI RNA. RNA-RNA interactions between the CI RNA and the target sites on the mRNA leader region are essential for transcri ption termination. To understand how transcription termination is elic ited by the P4 immunity mechanism, it is relevant to identify the tran scription termination site. This, however, could not be directly infer red from the 3'-end of the transcription products because of the exten sive and complex processing and degradation of the leader RNA. In this work, by making use of a tRNA gene as a reporter, we identify the ter mination site of the immunity transcripts (t(imm)). This is a Rho-depe ndent terminator located within the first translated gene of the left operon and is regulated by P4 immunity. Analysis of the P4 transcripti on pattern in Escherichia coli rho mutants suggests that termination a t t(imm) may also be important for the efficient processing of the CI RNA. (C) 1996 Academic Press, Inc.