Heat Shock Response
Heat shock response
The heat shock response occurs in virtually all organisms, including bacteria . The response occurs when an environmental stress is imposed on the organism. The name of the response comes from its discovery following the application of a mild heat stress, only 5 to 10° C higher than the usual preferred growth temperature. However, the response, which can be more correctly thought of as an adaptive response, consisting of a temporary alteration in the metabolism of the organism, occurs in response to more than just excessive heat.
Hallmarks of the heat shock response are its rapid onset and short-term nature. The response is an emergency coping type of reaction to a conditions that is perceived by the organism as being threatening. The response is not a long-term commitment, such as the formation of a spore by a bacterium (although some proteins that are stimulated in the heat shock response of Bacillus subtilis also function in the formation of spores by the bacteria). Rather, a heat shock response allows the organism to cope and then to quickly resume normal function.
The alteration in the chromosome of the fruit fly Drosophila flowing an elevation in the temperature was reported in 1962. At the time and for some years thereafter, the observation was regarded as an interesting curiosity that was relevant only to the fruit fly. However, it is unequivocally clear that the genes encoding the responsive proteins and the structure of the proteins themselves are highly conserved in many species. For example, three heat shock proteins that were discovered in bacteria, which have been dubbed Hsp70, Hsp10, and Hsp60 are highly conserved in a large number of bacteria and in eukaryotic organisms.
Heat shock proteins are induced in large quantities in response to factors including nutrient depletion, addition of alcohols such as ethanol, change in the sodium concentration, presence of heavy metals, fever, interaction with cells of a host, and the presence of virus. The proteins are produced in non-stress conditions. But typically their quantities are much less. In non-stress conditions they function in the normal metabolic events within the cell.
The heat shock response in bacteria involves the elevated production of more than 20 proteins whose functions are varied. For example, some of the proteins degrade other proteins (proteases), while other proteins help transport molecules from one place to another while preserving their structure (these are known as chaperones ). These induced proteins act to overcome the changes that would prove destructive to other proteins in the bacterium. By preventing protein alteration and destruction, the heat shock response ensure that the bacterium will be capable of normal function once the stress is removed.
Some bacteria also utilize the heat shock response to promote infection of host cells or tissue, or to survive within host cells. Examples of bacteria include Escherichia coli , Legionella pneumophila, and Listeria monocytogenes. Furthermore, the alteration in structure of bacteria can involve heat shock proteins. Examples include Bacillus subtilis spore formation and the formation of the so-called fruiting bodies by myxobacteria.
The principle trigger for the heat shock response in bacteria is at the level of transcription, where the genetic material, DNA (deoxyribonucleic acid ), is used to manufacture ribonucleic acid . In Escherichia coli the response is controlled by what has been called a sigma factor. The sigma factor is capable of binding to various regions of the DNA that stimulate the transcription of the particular gene under their control. In other words, the single sigma factor is capable of stimulating the expression of multiple genes. The sigma factor is under a tight and complex control, which normally restricts its activity but leaves the factor primed for action. This is the reason for the rapid nature of the heat shock response.
Two other heat shock response controls in bacteria operate after the proteins are produced. These controls aid in maintaining the proteins for a bit longer than if they were produced under non-heat shock conditions. By preserving the structure and functions of the heat shock proteins, their activity is allowed to persist. But, once again, the protein activity does not last indefinitely, which allows the heat shock response to be rapidly "turned off."
The observations that bacterial heat shock proteins can be vital for the establishment of infections has made the heat shock response the subject of much study, with the aim of circumventing the response or devising vaccines that protect host cells.
See also Bacterial adaptation; Microbial genetics