CAP binding site is located
CAP, also called CRP, binds as homodimer to 22-base pair inverted repeat containing TGTGA-N6-TCACA consensus motif located upstream of lac promoter, typically centered at position -61.5. When glucose scarce, adenylate cyclase synthesizes second messenger cAMP that binds N-terminal effector domains of CAP, allosterically enabling C-terminal helix-turn-helix domains to specifically recognize major groove sequences. Dimer introduces approximately ninety-degree bend in DNA, contacts alpha-carboxy-terminal domain of RNA polymerase via activating region 1, stabilizes closed promoter complex, and accelerates isomerization to transcriptionally competent open complex. Upstream position avoids overlapping operator, permitting simultaneous positive and negative regulation through distinct sites.
Ref: NCBI Bookshelf – Sigma; J Bacteriol 1997: CAP-cAMP binds 61.5 bp upstream lac promoter, Type I activation.