In Escherichia coli, cyclic AMP receptor protein (CRP) is known to regulate the transcription of about 100 genes. The signal to activate CRP is the binding of cyclic AMP. In this study the fluorescence quenching measurements were used to observe conformational changes in the structure of CRP after binding of cAMP and DNA. We used the constructed CRP heterodimer, which contains only a single Trp13 residue localized in the N-terminal domain of one CRP subunit. We propose that apo-CRP subunits exist in a solution in one conformational state and it changes after the ligand binding. We also suggest that the signal transmission upon binding of cAMP is possible not only from the N-terminal domain to C-terminal domain but also in the opposite direction after binding of specific DNA sequence, both with and without cAMP. Thereby it can influence on the CRP's interaction with RNA polymerase and the genes expression.