Master regulators of phenotypic differentiation in B. subtilis
In B. subtilis , the response regulators DegU, Spo0A, and ComA are recognized as the master switches that control the development of population heterogeneity . The activity of these three heterogeneity modulators depends on the ratios of the respective proteins in their non-phosphorylated and phosphorylated states (phosphorylated regulators are henceforth indicated with ∼P). In general, the phosphorylation state affects the regulator’s affinity for the promotor regions of the genes that they regulate . Delicate modulation of these ratios allows B. subtilis to develop a heterogeneous population, where cells adapt to small environmental differences (micro-niches) within the population, especially in the spatial structure of biofilms .
DegU is the response regulator of the DegS/U two component system. DegS is a cytoplasmic sensor histidine kinase that directly phosphorylates DegU. The DegU regulon is extensive and includes genes associated to motility (e.g. flagellum production) and biofilm formation (e.g. hydrophobin protein and exoenzymes needed for substrate degradation) . Non-phosphorylated DegU activates competence development, while depending on the relative amount of DegU∼P different sets of genes of its regulon are transcribed that provide a smooth transition from surface spreading to settlement during biofilm development .
Spo0A is a transcriptional regulator that controls the expression of hundreds of genes and operons in B. subtilis important among others for biofilm matrix production and the generation of spores . Spo0A is phosphorylated via a phosphorelay that can be initiated by any of five known independent histidine kinases (KinA, KinB, KinC, KinD, and KinE) . Once activated by their cognate signals, the Kin kinases phosphorylate the response regulator Spo0F, which in turn transfers the phosphoryl group to a secondary response regulator, Spo0B, which finally phosphorylates Spo0A. Like DegU, the amount of Spo0A∼P in the cells determines which of its target genes are expressed .
The third major master switch of population heterogeneity in B. subtilis , ComA, is directly modulated by a quorum sensing (QS) system . QS is a common cell-communication strategy that relies on the production and detection of extracellular autoinducer signaling molecules by cells of the same species . ComA-related QS system uses the ComX peptide as its autoinducer and the membrane localized ComP histidine kinase as sensor. The extracellular ComX signal activates ComP, leading to autophosphorylation and transfer of phosphate to ComA . Once phosphorylated, ComA controls the production of surfactin (an important surfactant lipopeptide), and the development of competence in B. subtilis cells .