Pre-sporulation stage -
Stress-inducing fluctuations in the environment, particularly limitations in nutrient availability, function as signals for bacterial adaptation. These signals are transferred into the cell via signal transduction systems and bring about cellular responses such as altered gene expression (described extensively in 1). One well-described signal transduction system for cellular developmental strategies of B. subtilis like biofilm formation and sporulation, is called the phosphorelay 2. Through the phosphorelay the key sporulation regulator Spo0A is phosphorylated and thus activated. During vegetative growth the expression of genes involved in these survival processes is inhibited via several different mechanisms. For instance, nutritional conditions causing high growth rates are reflected by increased levels of GTP, which are sensed and bound by CodY. This causes repression of spo0A transcription 3. This inhibition of sporulation is lifted when the cells enter stationary phase and GTP levels decrease - most likely due to accumulation of the stringent response 'alarmone' (p)ppGpp 4,5. Another signalling molecule called cyclic-di-AMP plays an important role in the timing of sporulation initiation 6.
Other key players in sporulation inhibition are the transition state regulators AbrB, SinR and ScoC 7. On the posttranslational level phosphorylation and dephosphorylation events of proteins control the levels of active (regulatory) proteins that, together with the transcriptional control, tightly regulate the cell's response to environmental signals 8.