A termination site for LacI transcription is between the CAP site and the lac promoter.

We have cloned a fragment of Escherichia coli DNA, extending from the end of the lacI gene into the lacZ gene, downstream from the trp promoter.This construction places the trp and lac promoters in tandem, with trp-promoted transcription proceeding through the end of the lacI gene in the normal direction.Transcription experiments performed in vitro with a purified DNA fragment carrying both promoters yielded three species of RNA.Two of these are the result of transcription to the end of the DNA template from initiations at the trp and lac promoters.The third species (RNA-2) is the product of trp-promoted transcription that terminates between the stop codon of the lad gene and the lac promoter.Fingerprint analysis and S1-mapping data localize the 3' end point of RNA-2 to a site approximately 50 base pairs distal to the lad stop codon.Efficiency of termination in vitro is a t least 20% and is unaffected by the addition of nusA protein or p factor.In addition, kinetic experiments demonstrate that a substantial pause is induced in the transcribing polymerase when it reaches this termination site.These characteristics suggest that this site probably functions in vivo as a terminator for lad-promoted transcription.The nucleotide sequence of this site reveals that despite little resemblance to the classic terminator structure (GC-rich dyad symmetry followed by sequence encoding a run of uridines), the region specifying termination bears a striking homology to two atypical factor-dependent terminators, rrnB tL and trp t'.Alignment of these two terminators with lad-t suggests a common evolutionary requirement among them.The lac operon has been studied extensively as a model system for the elucidation of structure and function of prokaryotic operons.A prominent feature is the phenomenon of negative control, whereby the product of the l a d gene (the lac repressor) prevents transcription of the lac operon.In lac, unlike most operons, the regulatory gene is located immediately upstream from the operon itself and is transcribed in the same direction.With only 84 bases between the termination codon of the lacl gene and the 5' end of ZacZ mRNA, this region displays a remarkably efficient use of DNA.It contains the recognition and binding sites for the CAMP receptor protein, RNA polymerase, and the lac repressor.Although a specific termination signal for lacl transcription in this region has not been hitherto identified, the existence of