Translations:Bacillus subtilis/12/en
Sporulation
Although sporulation in B. subtilis is induced by starvation, the sporulation developmental program is not initiated immediately when growth slows due to nutrient limitation. A variety of alternative responses can occur, including the activation of flagellar motility to seek new food sources by chemotaxis, the production of antibiotics to destroy competing soil microbes, the secretion of hydrolytic enzymes to scavenge extracellular proteins and polysaccharides, or the induction of ‘competence’ for uptake of exogenous DNA for consumption, with the occasional side-effect that new genetic information is stably integrated. Sporulation is the last-ditch response to starvation and is suppressed until alternative responses prove inadequate. Even then, certain conditions must be met such as chromosome integrity, the state of chromosomal replication, and the functioning of the Krebs cycle.
Once B. subtilis commits to sporulation, the sigma factor sigma F is secreted. This factor promotes sporulation. A sporulation septum is formed and a chromosome is slowly moved into the forespore. When a third of one chromosome copy is in the forespore and the remaining two thirds is in the mother cell, the chromosome fragment in the forespore contains the locus for sigma F, which begins to be expressed in the forespore. In order to prevent sigma F expression in the mother cell, an anti-sigma factor, which is encoded by spoIIAB, is expressed. Any residual anti-sigma factor in the forespore (which would otherwise interfere with sporulation) is inhibited by an anti-anti-sigma factor, which is encoded by spoIIAA. SpoIIAA is located near the locus for the sigma factor, so it is consistently expressed in the forespore. Since the spoIIAB locus is not located near the sigma F and spoIIAA loci, it is expressed only in the mother cell and therefore repress sporulation in that cell, allowing sporulation to continue in the forespore. Residual spoIIAA in the mother cell represses spoIIAB, but spoIIAB is constantly replaced so it continues to inhibit sporulation. When the full chromosome localizes to the forespore, spoIIAB can repress sigma F. Therefore, the genetic asymmetry of the B. subtilis chromosome and expression of sigma F, spoIIAB and spoIIAA dictate spore formation in B. subtilis.
Sporulation requires a great deal of time and also a lot of energy and is essentially irreversible, making it crucial for a cell to monitor its surroundings efficiently and ensure that sporulation is embarked upon at only the most appropriate times. The wrong decision can be catastrophic: a vegetative cell will die if the conditions are too harsh, while bacteria forming spores in an environment which is conducive to vegetative growth will be out competed. In short, initiation of sporulation is a very tightly regulated network with numerous checkpoints for efficient control.