The Importance of a Salt Bridge in FNR Transcription Factor Activity

Abstract

Fumarate Nitrate Reductase transcription factor (FNR) regulates the expression of genes involved in anaerobic respiration within E. coli bacteria. In the absence of oxygen, the FNR protein contains a [4Fe‐4S] cluster. This [4Fe‐4S] cluster causes a change in the dimerization helix of FNR, allowing the protein to dimerize and bind to DNA. In a recent publication, the crystal structure of A. fischeri FNR, which has high homology to E. coli FNR, was reported[1]. The structure suggests that the [4Fe‐4S] cluster promotes dimerization through a salt bridge interaction between residues Asp 130 and Arg 140. This investigation focuses the effect of the salt bridge interaction on the activity of E. coli FNR. Two mutant variants of FNR (Asp130Arg and Arg140Glu) were constructed in a pET11a plasmid containing the FNR gene. In each variant, one of the amino acid residues constituting the salt bridge was replaced with an oppositely charged amino acid side chain. The activity of the FNR mutant proteins was then measured under anaerobic conditions using the narG‐lacZ reporter strain. The results show that the activity of FNR decreases significantly when the salt bridge is broken. In addition, the results of the mutations that swap the amino acids of the salt bridge will be discussed.