The chromosomal inactivation of the unique transcription factor of Streptomyces coelicolor that displays a cyclic-nucleotide-binding domain, Crp(Sco), led to a germination-defective phenotype similar to the mutant of the adenylate cyclase gene (cya) unable to produce cAMP. By means of cAMP affinity chromatography we demonstrate the specific CAMP-binding ability of Crps, which definitely demonstrate that a Cya/cAMP/Crp system is used to trigger germination in S. coelicolor. However, electromobility shift assays with the purified Crp(Sco)-cAMP complex and the CRP-like cis-acting element of its own promoter failed. Moreover, we were unable to complement an Escherichia coli crp mutant in trans with Crps. The fact that Vfr from Pseudomonas aeruginosa and GIxR from Corynebacterium glutamicum could complement such an E. coli mutant suggests that the way Crp(sco) interacts with DNA should mechanistically differ from its most similar members. This hypothesis was further supported by homology modelling of Crps that confirmed an unusual organisation of the DNA-binding domain compared to the situation observed in Crp(Eco). (C) 2004 Elsevier Inc. All rights reserved.