The FNR family of transcriptional regulators

Abstract

Homologues of the transcriptional regulator FNR fromEscherichia coli have been identified in a variety of taxonomically diverse bacterial species. Despite being structurally very similar, members of the FNR family have disparate regulatory roles. Those fromShewanella putrefaciens, Pseudomonas aeruginosa, Pseudomonas stutzeri andRhodopseudomonas palustris are functionally similar to FNR in that they regulate anaerobic respiration or carbon metabolism. Four rhizobial proteins (fromRhizobium meliloti, R. leguminosarum, B. japonicum andAzorhizobium caulinodans) are involved in the regulation of nitrogen fixation; a fifth (fromRhizobium strain IC3342) has unknown function. Two proteins from mammalian pathogens (Actinobacillus pleuropneumoniae andBordetella pertussis) may be involved in the regulation of toxin expression. The FNR protein ofVibrio fischeri regulates bioluminescence, and the function of the one known FNR homologue from a Gram-positive organism (Lactobacillus casei) remains to be elucidated. Some members of this family, like FNR itself, appear to function as sensors of oxygen availability, whereas others do not. The ability to sense and respond to oxygen limitation may be correlated with the presence of cysteine residues which, in the case of FNR, are thought to be involved in oxygen or redox sensing. The mechanism of DNA sequence recognition is probably conserved, or very similar, throughout this family. In a number of other Gram-negative species, there is good indirect evidence for the existence of FNR analogues; these includeAlcaligenes eutrophus, A. denitrificans, A. faecalis, Paracoccus denitrificans and a number ofPseudomonas species.