Both prokaryotes and eukaryotes respond to a decrease in temperature with the expression of a speci®c subset of proteins. Although a large body of information concerning cold shock-induced genes has been gathered, studies on temperature regulation have not clearly identi®ed the key regulatory factor(s) responsible for thermosensing and signal transduction at low temperatures. Here we identi®ed a twocomponent signal transduction system composed of a sensor kinase, DesK, and a response regulator, DesR, responsible for cold induction of the des gene coding for the D5-lipid desaturase from Bacillus subtilis. We found that DesR binds to a DNA sequence extending from position ±28 to ±77 relative to the start site of the temperature-regulated des gene. We show further that unsaturated fatty acids (UFAs), the products of the D5-desaturase, act as negative signalling molecules of des transcription. Thus, a regulatory loop composed of the DesK±DesR two-component signal transduction system and UFAs provides a novel mechanism for the control of gene expression at low temperatures.
The parD operon of Escherichia coli plasmid R1 encodes a toxin–antitoxin system, which is involved in plasmid stabilization. The toxin Kid inhibits cell growth by RNA degradation and its action is neutralized by the formation of a tight complex with the antitoxin Kis. A fascinating but poorly understood aspect of the kid–kis system is its autoregulation at the transcriptional level. Using macromolecular (tandem) mass spectrometry and DNA binding assays, we here demonstrate that Kis pilots the interaction of the Kid–Kis complex in the parD regulatory region and that two discrete Kis-binding regions are present on parD. The data clearly show that only when the Kis concentration equals or exceeds the Kid concentration a strong cooperative effect exists between strong DNA binding and Kid2–Kis2–Kid2–Kis2 complex formation. We propose a model in which transcriptional repression of the parD operon is tuned by the relative molar ratio of the antitoxin and toxin proteins in solution. When the concentration of the toxin exceeds that of the antitoxin tight Kid2–Kis2–Kid2 complexes are formed, which only neutralize the lethal activity of Kid. Upon increasing the Kis concentration, (Kid2–Kis2)n complexes repress the kid–kis operon.
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