TrpI protein, the activator of transcription of the trpBA operon of fluorescent pseudomonads, bends the DNA when it forms either of two well-characterized complexes with the trpBA regulatory region. In complex 1, with TrpI bound only to its strong binding site (site I), the calculated bending angle is 65 to 67 degrees and the center of bending is in the middle of site I. In complex 2, which is required for activation of the trpBA promoter, with TrpI bound both to site I and to the weaker site II, the bending angle is increased to 89 to 90 degrees and the center of bending is at the site I-site II boundary. Indoleglycerol phosphate (InGP), which strongly stimulates formation of complex 2 and is required for activation, does not affect the bending angle of either complex. However, a mutation (-10C/11C) shown previously to affect activation has a small but detectable effect on bending, reducing the calculated bending angle to 83 to 86 degrees. These results suggest a way that DNA bending and InGP may be important for activation.