Sporulation in Bacillus

November 27, 2019

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.