Figure 8. T- or A-tracts, adjacent to −35 elements, regulate gene expression by a rheostat-like mechanism.

Schematic overview of the T-tract rheostat using the sabA promoter as a model. The predicted interaction of the RNA polymerase with sabA promoter, harboring different T-tract lengths and thereby different local DNA structure, is depicted in the model. The illustration shows the high-expressing T₁₃-variant and the low-expressing T₁₈-variant. The region containing the A-boxes, i.e. the proximal UP-like element, is marked in purple (−90 to −50), T-tract in blue, and the core promoter (−35 to +20) in yellow. Bent arrows indicate the change in local DNA structure that occurs in two orientations as the T-tract length is altered. This is a variable process as the T-tract length can both be lengthened and shortened, as a result of slipped strand mispairing during replication.