The leftward promoter of bacteriophage lambda. Structure, biological activity, and influence by adjacent regions.

The effect of regions adjacent to the lambda PL promoter was studied using a sequence deleted in an A/T-rich segment immediately upstream from the promoter. High resolution thermal denaturation analysis showed that the undeleted sequence, as isolated on a 360-bp restriction fragment (360-PL) melted in two distinct steps. Since the deleted sequence (230-PL) melts at a higher temperature than any portion of 360-PL, the deleted promoter is more stable to denaturation. This increased stability was also suggested by a transcription assay in which the strong binding of RNA polymerase is inhibited at low temperature. The deleted promoter fragment required a higher temperature to become functional than did the 360-PL fragment. Furthermore, RNA polymerase bound to the 230-PL fragment is displaced by heparin at a faster rate than from 360-PL, and the deleted fragment initiates transcription at a reduced rate. Thus, by several criteria the deleted promoter is less active than the parent sequence. Evidence is presented arguing that the deletion does not alter the sequence directly recognized by RNA polymerase. Therefore, the altered sequence is reducing promoter activity by changing some other level of overall promoter structure. These results suggest that regions outside of the sequence directly recognized by a regulatory protein can affect genetic expression from the site.