STRESS-INDUCED TRANSCRIPTIONAL ACTIVATION

Living cells, both prokaryotic and eukaryotic, employ specific sensory and signalling systems to obtain and transmit information from their environment in order to adjust cellular metabolism, growth, and development to environmental alterations Among external factors that trigger such molecular communications are nutrients, ions drugs and other compounds, and physical parameters such as temperature and pressure. One could consider stress imposed on cells as any disturbance of the normal growth condition and even as any deviation from optimal growth circumstances. It may be worthwhile to distinguish specific and general stress circumstances. Reasoning from this angle, the extensively studied response to heat stress on the one hand is a specific response of cells challenged with supra-optimal temperatures. This response makes use of the sophisticated chaperoning mechanisms playing a role during normal protein folding and turnover. The response is aimed primarily at protection and repair of cellular components and partly at acquisition of heat tolerance. In addition heat stress conditions induce a general response, in common with other metabolically adverse circumstances leading to physiological perturbations such as oxidative stress or osmostress. Furthermore, it is obvious that limitation of essential nutrients, such as glucose or amino acids for yeasts, leads to such a metabolic response. The purpose of the general response may be to promote rapid recovery from the stressful condition and resumption of normal growth. This review focuses on the changes in gene expression that occur when cells are challenged by stress, with major emphasis on the transcription factors involved, their cognate promoter elements and the modulation of their activity upon stress signal transduction. With respect to heat shock-induced changes, a wealth of information on both prokaryotic and eukaryotic organisms including yeasts, is available. As far as the concept of the general (metabolic) stress response is concerned major attention will be paid to Saccharomyces cerevisiae.