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PHOTOCHEMICAL CROSS‐LINKING OF THE CYCLIC ADENOSINE 3′, 5′MONOPHOSPHATE RECEPTOR PROTEIN TO Escherichia coli 5‐BROMOURACIL‐SUBSTITUTED DNA. ROLE OF THE EFFECTORS

Abstract— Cyclic AMP receptor protein (CRP) is a regulatory protein implicated in the transcription of several operons in Escherichia coli. Its activity is modulated by effectors, such as cAMP or cGMP, which could induce (or not) structural changes in the protein, and activate (or not) the transcription. CRP can bind non‐specifically to DNA, and we investigated the photocross‐linking of the protein to E. coli 5‐bromouracil‐substituted DNA, in the absence and in the presence of effectors. The photochemistry of the protein alone, under the conditions used for the cross‐linking reaction, was studied. We show that tryptophyl residues are more photoreactive than tyrosyl ones. Photocross‐linking of the protein implicates only one of the two subunits, and the rate of the reaction is not modified upon cAMP binding. Binding of cGMP reduces the rate of photocross‐linking by a factor of two. These new results show that the protein in the CRP‐cGMP complex behaves differently from the free protein.