抄録
It is well known that microorganisms can exist in every corner of natural environments, even if environments are extreme. As about 80% of biosphere in the earth's surface is 5℃ or colder, microorganisms must often encounter with cold environments. Most microorganisms may have evolved quite different ways of maintaining their membrane fluidity and metabolic activity constant, in response to changes in environmental temperature. The membrane fluidity is effectively altered by changes in fatty acid composition of lipids. After a shift-down in growth temperatures, the alteration of fatty acid composition such as increases in unsaturated, short-chain and branched fatty acids is mediated through the change in the activity of pre-existing enzymes and/or enzyme induction, depending on the species. By such means, microorganisms can regulate the activity of membrane-bound enzymes and transport systems. Simultaneously, a shift-down in growth temperatures induced increased rates of synthesis of more than a dozen proteins (so called cold-shock protein) in mesophiles, psychrotrophes and psychrophiles, in spite of the reduction in the rate of synthesis of most cellular proteins. Among these cold-shock proteins including seven proteins identified in E.coli, some are involved in transcription and translation. In the psychrophilic Vibrio sp., the temperature-related characteristics of protein synthesis in cells grown at 0℃ differed from those of cells grown at 13℃. This difference resulted from the modifications in the properties of the soluble fraction involved in protein synthesis. Such a modifications may be brought about by specific changes in the level of individual proteins whose the synthesis and degradation are induced by a shift in temperatures.
