Phylogeny of the bacterial superfamily of Crp-Fnr transcription regulators: exploiting the metabolic spectrum by controlling alternative gene programs

The Crp-Fnr regulators, named after the first two identified members, are DNA-binding proteins which predominantly function as positive transcription factors, though roles of repressors are also important. Among over 1200 proteins with an N-terminally located nucleotide-binding domain similar to the cyclic adenosine monophosphate (cAMP) receptor protein, the distinctive additional trait of the Crp-Fnr superfamily is a C-terminally located helix-turn-helix motif for DNA binding. From a curated database of 369 family members exhibiting both features, we provide a protein tree of Crp-Fnr proteins according to their phylogenetic relationships. This results in the assembly of the regulators ArcR, CooA, CprK, Crp, Dnr, FixK, Flp, Fnr, FnrN, MalR, NnrR, NtcA, PrfA, and YeiL and their homologs in distinct clusters. Lead members and representatives of these groups are described, placing emphasis on the less well-known regulators and target processes. Several more groups consist of sequence-derived proteins of unknown physiological roles; some of them are tight clusters of highly similar members. The Crp-Fnr regulators stand out in responding to a broad spectrum of intracellular and exogenous signals such as cAMP, anoxia, the redox state, oxidative and nitrosative stress, nitric oxide, carbon monoxide, 2-oxoglutarate, or temperature. To accomplish their roles, Crp-Fnr members have intrinsic sensory modules allowing the binding of allosteric effector molecules, or have prosthetic groups for the interaction with the signal. The regulatory adaptability and structural flexibility represented in the Crp-Fnr scaffold has led to the evolution of an important group of physiologically versatile transcription factors.