Homologues of the transcriptional regulator FNR from Escherichia 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 from Shewanella putrefaciens, Pseudomonas aeruginosa, Pseudomonas stutzeri and Rhodopseudomonas palustris are functionally similar to FNR in that they regulate anaerobic respiration or carbon metabolism. Four rhizobial proteins (from Rhizobium meliloti, R. leguminosarum, B. japonicum and Azorhizobium caulinodans) are involved in the regulation of nitrogen fixation; a fifth (from Rhizobium strain IC3342) has unknown function. Two proteins from mammalian pathogens (Actinobacillus pleuropneumoniae and Bordetella pertussis) may be involved in the regulation of toxin expression. The FNR protein of Vibrio 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 include Alcaligenes eutrophus, A. denitrificans, A. faecalis, Paracoccus denitrificans and a number of Pseudomonas species.