Oligomerization of Bacillus subtilis DesR is required for fine tuning regulation of membrane fluidity.

Background: The DesK-DesR two-component system regulates the order of membrane lipids in the bacterium Bacillus subtilis by controlling the expression of the des gene coding for the delta 5-acyl-lipid desaturase. To activate des transcription, the membrane-bound histidine kinase DesK phosphorylates the response regulator DesR. This covalent modification of the regulatory domain of dimeric DesR promotes, in a cooperative fashion, the hierarchical occupation of two adjacent, non-identical, DesR-P binding sites, so that there is a shift in the equilibrium toward the tetrameric active form of the response regulator. However, the mechanism of regulation of DesR activity by phosphorylation and oligomerization is not well understood.

Methods: We employed deletion analysis and reporter fusions to study the role of the N-terminal domain on DesR activity. In addition, electromobility shift assays were used to analyze the binding capacity of the transcription factor to deletion mutants of the des promoter.

Results: We show that DesR lacking the N-terminal domain is still able to bind to the des promoter. We also demonstrate that if the RA site is moved closer to the -35 region of Pdes, the adjacent site RB is dispensable for activation.

General Significance: Our results indicate that the unphosphorylated regulatory domain of DesR obstructs the access of the recognition helix of DesR to its DNA target. In addition, we present evidence showing that RB is physiologically relevant to control the activation of the des gene when the levels of DesR-P reach a critical threshold.