The ability of 24 systematically modified analogues of adenosine 3',5'-monophosphate (CAMP) to enhance the synthesis of j?-galactosidase in glucose-repressed Escherichia coli strains KNBL 1001 and cpd-Crookes has been investigated. The properties of the analogues in comparison with cAMP are, with only two exceptions, alike in both strains. Two analogues, 7-deazaadenosine 3',5'-monophosphate (i.e. tubercidin 3',5'-monophosphate) and (RJadenosine 3',5'-monothionophosphate, exhibit higher biological activity than CAMP. The latter analogue is 50-fold more active in both strains. Three analogues showed activities comparable to CAMP, four analogues were less active and 12 analogues were unable to antagonize catabolite repression. Structure-activity correlations showed that the 2'OH-, 3'0-, 5'0-, the negative charge and the 6-amino group cannot be modified without losing biological activity in uivo, while the N-1 and N-7 in adenine are not essential. The interaction with the catabolite gene activator protein is stereoselective for an unmodified axial exocyclic oxygen The molecular mechanism of CAMP action on stimulation of transcription of catabolite-sensitive gene operons was elucidated : a specific cAMP binding protein was isolated and purified. It was named 'catabolite gene activator protein' (CAP) [7] or, alternatively, 'CAMP receptor protein' (CRP) [8]. Regulation by CAP is exerted at the transcriptional level with cAMP acting as an allosteric effector [7,8]. cAMP forms a complex with CAP, thus inducing a conformational change [9], which results in specific recognition of DNA sequences near the promoter of several operons such as lac [lo], gal [ll], araC [12], and increases the rate of transcription initiation. In the lactose [lo] and arabinose operons [12] the CAMP-CAP complex functions as a positive regulator -it potentiates transcription. In the galactose operon, which has two overlapping promoters for RNA polymerase, the complex is required Dedicated to Prof. Dr Friedrich Cramer on the occasion of his 60th birthday.