Regulation of transcription from tandem and convergent promoters.

We have examined transcription on templates containing the trp and lac UV5 promoters arranged in tandem or opposing orientations. These studies have revealed that the strengths of the two promoters are comparable, though the lac UV5 promoter is much more sensitive to the level of initiating purine present. Kinetic experiments have shown that a polymerase molecule poised at the lac promoter, or a lac repressor molecule bound to the lac operator, can temporarily block a polymerase molecule initiated from the trp promoter, though transcription eventually continues through. In the convergent construct, transcription from the lac promoter is hindered only when initiation is suboptimal due to low purine concentrations.