Regulation of Sporulation

Regulation of Sporulation

	Regulation of Sporulation - In Bacillus subtilis the genes encoding the different steps in sporlation (spo genes) are widely dispersed and are cll1stered in about five chromosomal segments. It is presumed that these genes are repressed during vegetative growth. Analysis of the effects of mutations on the phenotype coded by the spo genes shows that sporulation is a 8equential process. There appears to be successive activation and repression of many operons. Experiments in which actinomycin D (an inhibitor of mRNA formation) is added at different stages of sporulation show that there are fairly long intervals between transcription of a particular mRNA and its translation to form a protein. For example, the mRNA for alkaline phosphatase is synthesized for about an hour before any alkaline phosphatase is detected in sporulating cells.
Similar time intervals between transcription and translation have also been observed with reference to refractility, synthesis of dipicolinic acid and heat resistance. It thus appears that regulation of endospore formation occurs at both transcriptional and translational levels.Radio autography and biochemical studies show that genes of both forespore and sporangium are active during sporulation. Some genes required for vegetative growth continue to be active, while others are switched off. Genes unique to sporulation are switched on. Since these genes are not active during vegetative growth, it must be assumed that they are repressed at this stage. Derepression must be taking place when vegetative growth ceases. Some triggering process must be involved for sporulation.One major regulatory mechanism is the change of specificity of RNA polymerase.
This is due to the decreased affinity of the sigma factor (u) for the core enzyme,and is probably due to an inhibitor. This is shown by the fact that sporulating cells lose the ability to transcribe the DNA of a phage which can multiply in vegetative cells. Triggers: Under conditions of deprivation the concentrations of several metabolites change. These changes may trigger sporulation. The purine metabolite adenosine bis-triphosphate appears early in sporulating cells, and may playa key role in sporulation. Sporulation is prevented by mutations, blocking the synthesis of this metabolite.
Regulation may also be brought about by one or more intracellular proteases. These enzymes appear early in sporulation and provide the cel1s with the amino acids required for synthesizing new proteins. Commitment to sporulation: Transfer of bacterial cells from a growing culture to a nitrogen-free medium induces massive sporulation. If nitrogen is provided for up to 2-3 hours after transfer, vegetative growth is resumed. After a certain period, however, the cells become committed to sporulation. If a nitrogen source is now added there is DO resumption of vegetative growth.
	The cells continue to sporulate. Commitment is the physiological point of no return. Experimental evidence indicates that there is no single point of commitment for the entire process of sporulation. Cells become successively committed to one sporulation event after another with initiation of sporulation. Thus commitment to alkaline phosphatase synthesis takes place early in sporulation, but not to refractility

When commitment to refractility later takes place, there is no simultaneous commitment to synthesis of dipicolinic acid.

Antibiotics:A regulatory role has been suggested for the antibiotics synthesized during sporulation.

Mutants in which gramicidin synthesis does not take place form defective heat-sensitive spores.

The formation of heat-resistant spores is restored on addition of gramicidin.

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