Complex two-component signaling regulates the general stress response in Alphaproteobacteria

Area of Science:

• Microbiology
• Molecular Biology
• Bacterial Physiology

Background:

• The general stress response (GSR) in Alphaproteobacteria is crucial for bacterial survival under adverse conditions.
• GSR activation is initiated by phosphorylation of the response regulator PhyR, leading to the release of sigma factor σ(EcfG).
• The specific signal transduction pathways controlling PhyR phosphorylation remain largely uncharacterized.

Purpose of the Study:

• To identify and characterize the components involved in the signal transduction pathways regulating PhyR phosphorylation in Sphingomonas melonis Fr1.
• To elucidate the roles of histidine kinases and single-domain response regulators in modulating the general stress response.
• To understand how different environmental stresses are integrated to activate the GSR.

Main Methods:

• Identification of seven HWE/HisKA2 family histidine kinases (PakA-G) and a single-domain response regulator (SdrG) in S. melonis Fr1.
• Phenotypic analyses and epistasis experiments to determine the functional relationships between identified components.
• In vitro phosphorylation assays to confirm direct interactions and regulatory mechanisms.

Main Results:

• Pak kinases directly phosphorylate PhyR and SdrG, acting as positive modulators of the GSR.
• SdrG functions upstream of or in parallel with PhyR, influencing its activity through a nonlinear pathway.
• Additional SDRRs negatively regulate the GSR, requiring the presence of both PhyR and SdrG.
• Analysis of thermal, osmotic, and oxidative stress responses revealed kinase redundancy and the ability of specific kinases to sense multiple stress conditions.

Conclusions:

• The alphaproteobacterial GSR is a complex, interconnected network involving multiple two-component systems converging on PhyR.
• PhyR phosphorylation is tightly regulated by both positive (Paks) and negative (SDRRs) modulators.
• The GSR integrates diverse stress signals, suggesting that bacteria perceive environmental conditions as combined cues rather than individual stresses.
• The findings suggest that HWE/HisKA2 kinases may broadly contribute to GSR regulation across Alphaproteobacteria.