Deletion analysis of the retroregulatory site for the lambda int gene.

The lambda int gene product, integrase, recombines phage and bacterial DNA at a specific site during the integration step of lysogeny. Regulation of integrase synthesis is complex. (1) Transcription of the gene can occur from either of two promoters. lambda cII protein activates transcription initiation near int at pI. The lambda N protein allows transcription of int from pL. N protein acts by preventing transcription termination at several terminators between pL and int. (2) The expression of integrase is also subject to post-transcriptional regulation by a site, sib, which is located beyond int in the b region of lambda. Expression of int from pL is inhibited by sib, whereas that from pI is not. The negative control of int expression by sib is termed retroregulation. Retroregulation of int is caused, in part, by processing of the pL transcript at the sib site by RNase III of Escherichia coli. The exonuclease, Bal31, was used to generate a set of deletions to define the sib regulatory site. Both sib+ and sib- deletions were sequenced, and it was concluded from this and other work that a dyad symmetry present in the b region, 270 base-pairs from int, was necessary for retroregulation. The RNA structure of this segment is similar to other RNase III-sensitive sites found in E. coli and phage RNAs.