FIG 7.

Graphs showing models for the effect of tandem promoters on the temporal expression of a gene. In both models, supercoiling-dependent transcription from P1 is upregulated in midcycle, when chlamydial DNA supercoiling levels are the highest, and transcription from P2 is repressed until late times by the transcription factor EUO. (A) Additive effect in which supercoiling-dependent transcription from P1 during midcycle is supplemented at late times by additional transcription from the late P2 promoter, leading to higher overall transcription of the gene (P1 + P2). (B) Alternative compensatory model in which P1 transcription decreases at late times in response to lower supercoiling levels but is replaced by late P2 transcription, which maintains overall transcription (P1 + P2).