Isolation and characterization of the Fnr protein, the transcriptional regulator of anaerobic electron transport in Escherichia coli.

The Fnr protein, the transcriptional regulator of the expression of anaerobic respiratory functions in Escherichia coli, was purified to homogeneity from soluble extracts of a strain harbouring the fnr gene in an expression vector. The identity of the isolated protein was confirmed by comparing its amino-terminal sequence with that predicted from nucleotide sequence of the fnr gene. It appeared that eight amino-terminal amino acids had been removed post-translationally from the bulk of the isolated Fnr protein. The molecular mass of the isolated protein (Mr 28 000) was consistent with a monomeric state, but sedimentation coefficients for the cellular (4.1 S) and the isolated (2.9 S) Fnr protein suggest that it may exist as a dimer in the bacterial cells. The Fnr protein bound DNA. However, the binding activity was not specific for the regulatory regions of relevant genes and it could not be stimulated by a variety of conditions or potential effectors. Two of the four cysteine residues of the Fnr protein were alkylated by iodoacetic acid and this could have functional significance in rendering the protein redox-sensitive.