Removal of a terminator structure by RNA processing regulates int gene expression

The int gene of phage λ encodes a protein involved in site-specific recombination. Its expression is regulated differentially during successive phases of the λ infective cycle. The gene is transcribed early after infection from one promoter, p L, and later from a second promoter p I. Each transcription event requires different positive activation factors, λ N and cII proteins, respectively. Transcription from the p I promoter, located adjacent to int, passes through int and terminates 277 nucleotides beyond int at t I. Polymerases initiating at p L transcribe through t I, and into the b segment of λ DNA. The read-through p L transcript is sensitive to cleavage by the endonuclease, RNase III, both in vivo and in vitro. Two specific cuts are made by RNase III in a double-stranded structure about 260 nucleotides beyond int in the location of the t I terminator. Functionally, the processed p L transcript is unable to synthesize the int gene product, whereas the terminated and unprocessed p I transcript expresses int. Interestingly, unprocessed p L transcripts made in hosts defective in RNase III ( rnc −) can express int. Thus a correlation exists between processing and negative control of int expression. The place where processing occurs, some 260 nucleotides beyond int, is called sib, and the control of int expression from this site is called retroregulation. Retroregulation by sib is not restricted just to the int gene; we show that if the sib site is cloned beyond a bacterial gene, the gene is controlled by sib and RNase III. Specific models are discussed with respect to control of gene expression by RNase III from a site beyond the controlled gene.