Cascades of Alternative Sigma Factors Occur in Bacillus Spore Formation

The requirement of RpoE for transcription of the rpoH gene illustrates, at a simple level, that the expression of one sigma factor may depend upon another. Indeed, in some complex processes, a series of alternative sigma factors may depend on each other. The classic case of a cascade of alternative sigma factors is the regulation of protease Enzyme that degrades proteins

FIGURE 9.04 Regulation of the Heat Shock Response

At normal temperatures, the RpoH sigma factor binds the chaperonin and is presented to the protease for digestion. At high or extreme temperatures, the chaperonin binds instead to misfolded proteins, leaving RpoH free to activate the heat shock genes.

FIGURE 9.04 Regulation of the Heat Shock Response

At normal temperatures, the RpoH sigma factor binds the chaperonin and is presented to the protease for digestion. At high or extreme temperatures, the chaperonin binds instead to misfolded proteins, leaving RpoH free to activate the heat shock genes.

spore formation in the gram-positive bacterium, Bacillus. When nutrients are scarce, Bacillus forms spores designed to survive bad times. Spores are formed by an asymmetric division that gives a full-sized mother cell and a much smaller spore. The spore is surrounded by the mother cell until it is fully developed. The mother cell then bursts, releasing the spore. This represents cellular differentiation at its most primitive level (Fig. 9.05 and Fig. 9.06).

Spore formation is controlled by four alternative sigma factors, oE and oK in the mother cell, and, once sporulation has started, oF and oG in the developing spore. Two of these, oE and oK, are first synthesized as inactive precursor proteins—pre-oE and pre-oK that must be activated by specific proteases. First, an environmental signal activates the synthesis of pre-oE in the mother cell and oF in the spore. The presence of oF allows transcription of early sporulation genes in the spore. These include the gene for the sigma factor, oG, as well as protein sporulation factors that move into the mother cell and split pre-oE protein to give active oE. The activated oE switches on spore A cell specialized for survival under adverse conditions and/or designed for distribution

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