- Many microorganisms including prokaryotes and eukaryotes respond to adverse conditions by forming spores.
- When the good condition returns once, the spore germinates and the microorganism returns to its normal life cycle.
- Among the bacterial group, the genus Bacillus is well known for the formation of endospores which means the spores formed inside the mother cell.
- Prior to endospore formation, the cell divides asymmetrically.
- The smaller cells develop into the endospore which is usually surrounded by the large mother cell.
- Once the development is completed, the endospores are released by the bursting of mother cell.
- Endospore formation in Bacillus is generally triggered by the adverse external conditions like starvation, desiccation, or growth-inhibitory temperatures.
- Infact, multiple aspects of environment get monitored by a group of five sensor kinases.
- These function via a phosphotransfer relay system whose mechanism is similar to two-component regulatory system but is considerably a complex process.
- The net result of multiple adverse conditions is the successive phosphorylation of several proteins which are called sporulation factors, culminating with the sporulation factor SpoOA.
- When SpoOA is highly phosphorylated, sporulation process gets initiated.
Development of the endospore
- Once triggered, endospore development is under the control of four different sigma factors, two of which (σF and σG) activates genes needed inside the developing endospore itself.
- Remaining two (σE and σK) are responsible for activating genes needed in the mother cell that surrounds the endospore.
- The sporulation signal transmitted via SpoOA activates σF in the smaller cell that is destined to become the endospore.
- σF is already present but is in inactive form due to binding with anti-sigma factor.
- The signal from SpoOA activates a protein that binds to the anti-sigma factor.
- The anti-sigma factor then gets inactivated and liberates σF.
- Once free, σF binds to RNA polymerase and allows transcription (inside the spore) of genes whose products are essential for the next stage of sporulation.
- These include the gene for the sigma factor σG and the genes for proteins that cross into the mother cell and activate σE.
- Active σE is required for transcription inside the mother cell of yet more genes, including the gene for the sigma factor σK.
- The sigma factors σG (in the endospore) and σK ( in the mother cell) are required for transcription of genes which are needed in the later stages of sporulation process.
- One fascinating aspect of endospore formation is that it is preceded by what is in effect cellular cannibalism.
- Those cells where SpoOA has already been activated secrete a protein that lyses nearby cells of the same species whose SpoOA protein has not been activated.
- This toxic protein is accompanied by a second protein that delays sporulation of neighboring cells.
- Cells that are committed to sporulation also make antitoxin protein to protect themselves against the effects of their own toxin.
- Their sacrificed sister cells are used as a source of nutrients that is required by developing endospores.
- Shortages of certain nutrients such as phosphate increase the expression level of the toxin-encoding gene.
Sporulation in Bacillus