FIG 4.
Consequences of relaxation of the promoter DNA bend angle upon effector binding (blue circle) to CbbR. For four CbbR-containing bacteria, a change in the promoter DNA bend angle has been shown to occur upon effector binding to the effector pocket of the protein. In R. sphaeroides, as illustrated here, CbbR undergoes a conformational change upon binding of a positive effector such as RuBP as a result of switching from heterotrophic to photo- or chemoautotrophic CO2-dependent growth conditions, thus allowing a relaxation of DNA bending and subsequent transcription of the cbb genes. Under appropriate physiological conditions, RuBP concentrations greatly increase and RuBP binds to CbbR of R. sphaeroides (76) and CbbRI of R. capsulatus (84), while phosphoenolpyruvate (PEP), FBP, and 3-PGA concentrations greatly increase and each binds to CbbRII of R. capsulatus during photoautotrophic growth (85). NADPH concentrations rapidly increase in X. flavus (83, 87) and H. thermoluteolus (8) during chemoautotrophic growth. This relaxation of the DNA bend angle leads to appropriate contact with RNA polymerase and activates transcription of the cbb operon. RBS, recognition binding site; ABS, activation binding site. RD-1 and RD-2, recognition domains 1 and 2, respectively.