GcvA and Lrp are both necessary for activation of the gcv operon. The upstream GcvA-binding sites 3 and 2 were separated from the Lrp-binding region and the rest of the gcv control region. Moving these sites by 1 or 2 helical turns of DNA further from the gcv promoter reduces, but does not eliminate, either GcvA-mediated activation or repression of a gcvT::lacZ gene fusion. However, moving these sites by 1.5 or 2.5 helical turns of DNA results in a GcvA-mediated super-repression of the operon. This repression is dependent on Lrp and is partially dependent on GcvR. Lrp bound to the gcv control region induces a bend in the DNA. Based on these results, a model for gcv regulation is presented in which Lrp plays a primarily structural role, by bending the DNA and GcvA functions as the activator protein.
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