Molecuar model of the DNA interaction site for the cyclic AMP receptor protein.

A topological model of the DNA binding site for the cyclic AMP receptor protein (CRP) is presented. A consensus sequence drawn from the known CRP binding sites has several symmetrical subregions that are spatially resolved onto different faces of the DNA helix. Consideration of available biochemical and genetic data suggest one particular choice among the possible symmetrical arrangements. In this case, the sequence of its helical form presents nearly the same pattern of exposed base pairs on two faces of the helix. These two faces are separated by a helix angle of 72 degrees; the similar sequences that are exposed in the grooves occur in opposite orientations on the two faces. We propose that this inverted symmetry arrangement provides each of the identical subunits of the CRP with a similar recognition region within the overall site. In gal and ara, the site appears to accommodate a single molecule of the CRP; in lac, the site repeats the symmetrical arrangement and should accommodate two molecules of the CRP.