Molecular determinants of negative regulation of the Bradyrhizobium diazoefficiens transcription factor FixK2

Abstract

FixK2 is a key regulator involved in the control of different lifestyles of the facultative soybean endosymbiont Bradyrhizobium diazoefficiens. Expression of the fixK2 gene is activated by the superimposed two-component regulatory system FixLJ in response to low oxygen (at or below 5 % oxygen in the gas phase) and repressed by its own product (directly or indirectly) by a still unknown mechanism. FixK2 is an unusual member of the CRP/FNR family of bacterial transcription factors, because it is active in vitro without an additional effector molecule and is negatively regulated posttranslationally by the oxidation of its singular cysteine residue (Cys-183). FixK2 is also regulated by proteolysis by the ClpAP1 chaperone-protease system. Remarkably, the recently solved FixK2 structure in complex with DNA revealed the molecular basis for the posttranslational control of FixK2. Specifically, both Cys-183 and the last 12 amino acids of FixK2 are surface-exposed, rendering the protein accessible for both oxidation and proteolysis, respectively. Here, we compile the updated knowledge about the critical elements involved in the negative control of fixK2 gene expression and FixK2 protein activity.