The binding of the cyclic AMP receptor protein (CRP) to symmetrical synthetic DNA-binding sites was investigated with a gel-retardation assay. A set of ten different sequences was employed, comprising all base permutations at positions 2, 4, and 5 of the consensus sequence 5'(TGTGA)3'. We show that: (i) CRP has a higher affinity for the completely symmetrical site than towards the lac wild-type site; (ii) base substitutions at position 2 lead to either a complete loss of specific CRP binding (G----C), a reduction in specific CRP binding (G----A) or only marginal effects on specific CRP binding (G----T); (iii) changes at position 4 abolish (G----C; G----A) or reduce (G----T) specific CRP binding; and (iv) base permutations at position 5 reduce specific CRP binding, but never completely abolish it. Thus position 4, and to a lesser extent position 2, in the DNA consensus sequence are the most crucial ones for specific binding by CRP.