Architecture of the complete oxygen-sensing FixL-FixJ two-component signal transduction system

Abstract

The symbiotic nitrogen-fixing bacterium is critical to the agro-industrial production of soybean because it enables the production of high yields of soybeans with little use of nitrogenous fertilizers. The FixL and FixJ two-component system (TCS) of this bacterium ensures that nitrogen fixation is only stimulated under conditions of low oxygen. When it is not bound to oxygen, the histidine kinase FixL undergoes autophosphorylation and transfers phosphate from adenosine triphosphate (ATP) to the response regulator FixJ, which, in turn, stimulates the expression of genes required for nitrogen fixation. We purified full-length FixL and FixJ proteins and defined their structures individually and in complex using small-angle x-ray scattering, crystallographic, and in silico modeling techniques. Comparison of active and inactive forms of FixL suggests that intramolecular signal transduction is driven by local changes in the sensor domain and in the coiled-coil region connecting the sensor and histidine kinase domains. We also found that FixJ exhibits conformational plasticity not only in the monomeric state but also in tetrameric complexes with FixL during phosphotransfer. This structural characterization of a complete TCS contributes both a mechanistic and evolutionary understanding to TCS signal relay, specifically in the context of the control of nitrogen fixation in root nodules.

Oxygen triggers signal transduction in the DevS (DosS) sensor of Mycobacterium tuberculosis by modulating the quaternary structure

“…By contrast, oxy-DevS as well as oxy-ReFixL deviated from this trend, possibly because of a more elongated structure for the inactive states. Structural studies of FixL from Bradyrhizobium japonicum (BjFixL) using SAXS and DLS also indicate an elongated structure [55]. The efficient crosslinks involving the phenolic and amine side chains (Tyr and Lys, N-term), which reside mainly in the DevS heme-binding domain, suggest that this domain might be a key interface for the oligomerization (Fig.…”

“…By contrast, oxy‐DevS as well as oxy‐ Re FixL deviated from this trend, possibly because of a more elongated structure for the inactive states. Structural studies of FixL from Bradyrhizobium japonicum (BjFixL) using SAXS and DLS also indicate an elongated structure .…”

“…Among them, FixL has been the most widely investigated, and this has provided the basis for many studies in the field. Although interesting mechanistic features have been unraveled, including a hysteresis effect on signal transduction, and small angle X‐ray scattering (SAXS) studies have been conducted, the structural mechanisms for signal transduction are not yet understood .…”

“…Among them, FixL has been the most widely investigated, and this has provided the basis for many studies in the field. Although interesting mechanistic features have been unraveled, including a hysteresis effect on signal transduction, and small angle X-ray scattering (SAXS) studies have been conducted, the structural mechanisms for signal transduction are not yet understood [19,21,36,55]. The most commonly proposed structural mechanism of regulation for the kinases is a conformational change starting at the heme that might influence domain-domain interactions.…”

Basic Biology of Hypoxic Responses Mediated by the Transcription Factor HIFs and Its Implication for Medicine

“…In the simplest model, the sensor binds oxygen directly, causing the fraction of sensor molecules bound to ligands to decline with the oxygen concentration. The bacterium, Rhizobium meliloti, exhibits a two-component signaling system consisting of FixL, a hemoprotein kinase that is active in the deoxy state, and FixJ, a transcription factor that becomes active when phosphorylated by FixL [38,39]. Hemoglobin also acts as an O 2 sensor.…”