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Starvation – activation of Spo0A |
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Starvation – activation of Spo0A
In response to nutrient limitation stress, several stress response mechanisms are activated within the bacterial cell. The cells can become motile in the search for new nutrients or form biofilms or spores in order to protect themselves against harsh conditions. These cellular responses are not uniform, even in an isogenic culture. Due to molecular stochasticity (e.g. differences in regulatory protein levels) and complex regulatory circuits, cells within a population can respond differently and have various fates, with sporulation often considered to be a last resort survival strategy 8,29,30,31.
The initial entry into sporulation is cell cycle and cell density dependent 32. It starts with the sensing of nutrient limitations in the environment and is followed by transfer of response signals into the cell to stop cell proliferation (reviewed in33,34). Importantly, sporulation is a very energy-consuming process, which is shown to be irreversible from stage II onwards 35 and cannot proceed in the complete absence of nutrients 36. The sensing and signal transfer is mediated by sporulation-specific two-component switches and the multicomponent phosphorelay 37. The combined action of autophosphorylation of activated kinases and signal transduction in the form of phosphate flow results in the phosphorylation, dimerization and subsequent activation of the sporulation key regulator Spo0A 38. This is known to be a heterogeneous event due to stochastic fluctuations in the phosphate flow and the presence of a bistable switch 39, causing different levels of activated Spo0A throughout the population 40,41. Furthermore, a cascade of events leading to inhibition of dephosphorylating Rap proteins at high cell density levels ensures the accumulation of phosphorylated Spo0A (reviewed in 33). Once activated, Spo0A~P binds to the so-called 0A boxes upstream of approximately 120 genes, including abrB, sinI and spo0A to repress or stimulate their transcription 42. Spo0A~P levels are gradually increased 43, which greatly determines the eventual cellular response caused by the activation or repression of low- and high threshold genes within the Spo0A~P regulon 44. For instance, production of killing factors and sporulation-delaying protein when Spo0A~P levels are still low, limits the commitment of sporulation within the population 45.
Simultaneously, abrB transcription is repressed, which lifts the inhibiting action of this repressor on sporulation 46. In addition, the sinI gene is expressed to give rise to a SinR antagonist that inhibits the repressing action of SinR on genes involved in biofilm formation and sporulation processes 18,47. In conjunction with specific sigma factors σA and σH sufficient levels of Spo0A~P are responsible for the first cellular responses to the nutrient limiting environment.
Phosphorelay
The main function of the phosphorelay is to sense and respond to environmental stimuli and to transfer the response signal in the form of phosphoryl flux throughout the relay. Within this process five kinases (KinA – KinE) are involved that are subjected to autophosphorylation. Despite repeated efforts, the exact stimuli to activate these kinases are still unknown. A recent publication, however, describes pyruvate, propionate and butyrate as possible ligands for the KinD kinase 48, providing new leads for identification of signals that cause entry into sporulation.
Transfer of the phosphoryl groups by the kinases to the response regulator Spo0F and phosphotransferase Spo0B eventually results in the phosphorylation and activation of Spo0A (reviewed in 49,50). Several phosphatases that dephosphorylate Spo0A~P form part of an important system that controls the levels of phosphorylated Spo0A. The alternative sigma factor σB, known for its regulation of general stress responses, was shown to be involved in the control of Spo0A~P levels through the activation of one of the phosphatase encoding genes, spo0E 51. This indicates that not one specific, but rather several regulatory systems are responsible for establishing appropriate conditions for commitment to sporulation.
Recently, several independent reports have indicated that the origin of heterogeneity in the sporulation process can be traced back to dynamics in the regulation of individual phosphorelay components 41,52,53,54,55 and that pulsed positive feedback loops involving Spo0A phosphorylation and kinase expression are essential for timing of commitment to sporulation 56.