Temperature Effects on Biological Systems: Introduction.
Science Progress 2003, Spring-Summer
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This issue begins with an excellent and comprehensive article on cold-shock responses by Weber and Marahiel (1). These authors review in detail the changes which occur on transfer of bacteria to low temperatures, with major sections on regulatory aspects of response induction, the function of the different components of the response, and the major physiological changes observed in the shocked organisms. Their main emphasis, however, is on cold perception, particularly covering how low temperatures alter structural and synthetic components, and how these changes could trigger response induction. Three particular structures/components are considered, in detail, as possible cold sensors, namely the membranes, the DNA and the ribosomes. Changes to the membrane fluidity, for example, on temperature down-shift, could form one of the primary cold perception signals, switching-on cold responses, and similar membrane alterations may be involved in cold-induced changes to chemotactic behaviour. Since changes to DNA sup ercoiling are known to occur at low temperatures, this molecule, associated with proteins which modulate its functioning, is also considered as a cold perception system (1), probably switching-on a group of components distinct from that governed by membrane changes. Further cold-shock processes may be initiated following changes to ribosome functioning (1 2) at low temperatures, possibly involving reduced (p)ppGpp levels. Coldinduced changes to other molecules, such as m-RNAs (3) could also be involved in cold-sensing. Although it is clear from the above article (1), that the mechanisms controlling many of the stages in cold-shock response induction are well studied and understood, and that certain structural and synthetic systems may function as cold sensors, there is one area, relating to such cold perception, which has been overlooked in the literature, namely whether cold-exposed organisms can, by use of diffusible extracellular pheromones, give early warning to non-exposed organisms. All mechanisms proposed for cold sensing' involve the low temperatures affecting the properties or structures of internal or surface components, such as membranes', DNA', ribosomes (1,2) or m-RNAs (3). There is, however, a considerable problem with such sensing mechanisms, since even if organisms are in a region close to one which is being subjected to cold stress, only when the temperature falls sharply in the immediate area of these organisms, will coldshock response induction begin, and such induction will be very slow, because the orga nisms will be, by then, at very low temperatures. For rapid induction, we would need switching-on to occur prior to exposure to cold, because only then would the response occur at an appreciable temperature (and, therefore, at an appreciable rate). We now know that most stresses, including physical ones, are sensed by extracellular components, namely extracellular sensing components, ESCs (4), which are converted by the stress to extracellular induction components, EICs, which then allow organisms to be warned of impending stresses (Table 1). Response induction prior to exposure to cold stress, could occur if such an ESC detected the fall in temperature, and the EIC which derived from it then diffused to unstressed regions i.e. the cold-related EIC would act as an extracellular alarmone and pheromone, allowing intercellular communication between cold-exposed and non-exposed organisms. This would solve the above problem of painfully slow induction of coldshock responses, since such diffusion from regions of co ld-stress to nearby regions, which were about to be stressed, would induce the responses. Because these latter regions would not yet have reached low temperatures, response induction would be much more rapid (Table 1). So far, the occurrence of cold shock-related ESCs and EICs has not been proven, but Nikolaev (5) has shown that filtrates from cold-shocked cultures can protect organisms from stres
- 2,99 €
- Category: Engineering
- Published: 22 March 2003
- Publisher: Science Reviews Ltd.
- Print Length: 11 Pages
- Language: English