Almost all organisms that have been studied respond to heat shock, so this environmental condition can work well as an example. Many of the genes expressed at high temperatures are highly conserved throughout evolution. However, the regulatory circuits that control the heat shock genes vary greatly from one group of organisms to another. In bacteria such as E. coli, two alternative sigma factors, RpoH and RpoE, control the heat shock response.
At increasingly high temperatures, the 3-D structure of proteins begins to unravel. Unfolded or misfolded proteins not only lose their own activity, but they may also bind to other functional proteins and create insoluble aggregates. Thus, the heat shock response is primarily concerned with protecting the cell from damaged and/or mis-folded proteins.
E. coli is optimized for growth at body temperature (37°C). It grows happily up to about 43°C but almost stops growing at 46°C. At 46°C, about 30 percent of all the proteins made by E. coli are heat shock proteins. Most of these fall into two categories: some are chaperonins that help other proteins fold correctly and prevent aggregation, alternative sigma factor A nonstandard sigma factor needed to recognize a specialized subset of genes chaperonin A protein that helps other proteins fold correctly heat shock proteins A set of proteins that protect the cell against damage caused by high temperatures heat shock response Response to high temperature by expressing a set of genes that encode heat shock proteins
A) Repair, if possible
B) Destroy, if cannot repair
Protein too damaged to repair
Properly folded protein released
Damaged protein pulled in
FIGURE 9.03 Heat Shock Response in E. coli
The cell responds to heat shock either by repairing misfolded proteins or by degrading them.
while others are proteases that degrade heat-damaged proteins that are past rescue (Fig. 9.03).
When the environmental temperature of cells is increased from 30°C to 43°C, the level of RpoH rises. This results in elevated expression of the heat shock genes that depend on RpoH for transcription. Control of the level of RpoH itself is very complex and modulated by a variety of minor factors; however, the main signal is the level of misfolded proteins present in the cell. This is monitored by two heat shock proteins, DnaK (a chaperonin) and HflB (a protease). When the level of misfolded proteins is low, DnaK and HflB are free and they apparently bind to RpoH and degrade it. In addition, they bind to partly synthesized RpoH protein, even before it is finished by the ribosome, and block further translation. When the level of misfolded proteins rises, DnaK and HflB bind to these and are unable to affect RpoH levels (Fig. 9.04).
Transcription of the rpoH gene from its main promoter requires the standard sigma factor o70 or RpoD. At temperatures above 50°C, o70 is inactivated and synthesis of RpoH would come to a halt, thus undermining the heat shock response. This is prevented by the presence of a second promoter for the rpoH gene that can be recognized by the RpoE sigma factor. Transcription can continue until 57°C, when the core enzyme of RNA polymerase is inactivated.
The level of RpoE (E for extra-cytoplasmic) is controlled in response to the level of misfolded proteins in the outer membrane and periplasmic space, rather than in the cytoplasm, as in the case of RpoH. In addition to rpoH, another group of a dozen or so heat shock genes requires RpoE for their transcription.
Some bacteria survive hard times by making spores.
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