Bacillus subtilis DesR Functions as a Phosphorylation-activated Switch to Control Membrane Lipid Fluidity

Abstract

The Des pathway of Bacillus subtilis regulates the synthesis of the cold-shock induced membrane-bound enzyme ⌬5-fatty acid desaturase (⌬5-Des). A central component of the Des pathway is the response regulator, DesR, which is activated by a membrane-associated kinase, DesK, in response to a decrease in membrane lipid fluidity. Despite genetic and biochemical studies, specific details of the interaction between DesR and the DNA remain unknown. In this study we show that only the phosphorylated form of protein DesR is able to bind to a regulatory region immediately upstream of the promoter of the ⌬5-Des gene (Pdes). Phosphorylation of the regulatory domain of dimeric DesR promotes, in a cooperative fashion, the hierarchical occupation of two adjacent, non-identical, DesR-P DNA binding sites, so that there is a shift in the equilibrium toward the tetrameric active form of the response regulator. Subsequently, this phosphorylation signal propagation leads to the activation of the des gene through recruitment of RNA polymerase to Pdes. This is the first dissected example of a transcription factor functioning as a phosphorylationactivated switch for a cold-shock gene, allowing the cell to optimize the fluidity of membrane phospholipids.All organisms must communicate with their environment to survive. Bacterial cells monitor external conditions and process this information to give the most appropriate response. Twocomponent regulatory systems have emerged as a paradigm for adaptive responses. In its simplest form, a two-component system contains a sensor (histidine kinase) and a response regulator (often a transcriptional activator) (1). Changes in the environment result in phosphorylation of the sensor followed by transphosphorylation onto the response regulator (1, 2). Although kinase-response regulator pairs of this type were frequently reported as governors of a wide variety of pathways in response to a myriad of signals (3-5), the requirement of this system to control gene expression during cold-shock has only recently been discovered in Bacillus subtilis (6) and cyanobacteria (7).Cold shock is a stress condition that adversely affects the growth of poikilothermic organisms, such as bacteria and plants. Thus, understanding the mechanisms by which these organisms perceive low temperature signals and transmit this information to the cellular machinery to activate adaptive responses is of fundamental importance to biology.Bacteria and most (if not all) poikilothermic organisms have to remodel the membrane lipid composition to survive at low temperatures. B. subtilis (6) and Synechocystis (7) respond to a decrease in the ambient growth temperature by introducing double bonds into the acyl chains of their membrane phospholipids by membrane-bound acyl lipid desaturases. This postsynthetic modification of the saturated acyl chains seems to be designed to ameliorate the effect of temperature changes on the physical state of membrane phospholipids. In Synechocystis it was found that inactivation of two histidine kinases mod...

Adaptation and Acclimation of Photosynthetic Microorganisms to Permanently Cold Environments

“…In the cyanobacterium genus Synechocystis, the expression of three desaturases (encoded by desA, desB, and desD) is inducible under cold temperatures. The sensor/signal pathway for regulation of transcription of the single desaturase found in Bacillus subtillus, encoded by des, has also been thoroughly investigated (35,142).…”

Phosphatidylethanolamine Is a Key Regulator of Membrane Fluidity in Eukaryotic Cells

“…In bacteria, membrane fluidity is mostly regulated by adapting the length and saturation of acyl chains (3). For instance, upon lowering of the growth temperature, bacteria will typically increase the content of unsaturated acyl chains, shorten chain length, and modify branching or cyclization to lower the temperature of phase transition from gel to liquid-crystalline state, a process called homeoviscous adaptation (4).…”

Structural plasticity and catalysis regulation of a thermosensor histidine kinase

“…Several lines of evidence show that the cold thermal stimulus is detected by DesK, which together with the response regulator (RR), DesR, constitutes a canonical two-component system (TCS). In vivo experiments have demonstrated that DesK acts as a kinase at cold temperatures (4), ultimately activating the transcription of the gene des coding for the acyl lipid desaturase ⌬5-Des (1,5). The increased fraction of unsaturated fatty acids in the membrane then restores fluidity and shuts off the kinase activity of DesK, terminating transcription.…”