All organisms respond to heat by inducing the synthesis of a group of proteins called the heat-shock proteins or hsps. The response is the most highly conserved genetic system known, existing in every organism in which it has been sought, from archaebacteria to eubacteria, from plants to animals. Although certain features of the response vary from organism to organism, many are universal, or nearly so. All organisms examined produce proteins encoded by the hsp70 and hsp90 gene families in response to elevated temperatures. These proteins are among the most highly conserved proteins in existence. Also universally, several of the proteins induced by heat are induced by a variety of other stresses. Although the particular constellation of effective inducers varies somewhat from organism to organism, in nearly all cells anoxia, ethanol, and certain heavy metal ions induce the proteins. Furthermore, either the hsps themselves or their close relatives are present in all organisms at normal temperatures and play vital roles in normal cell function. This last finding has providcd important information on the specific molecular functions of the proteins and will be discussed in detail in this reVIew.

An early and long-standing assumption about the heat-shock response is that the hsps protect cells from the toxic effects of heat and other stresses; good evidence supports this view. There is, first of all, the very nature of the response. In all organisms, the induction of hsps is remarkably rapid and intense, in keeping with the notion that it is an emergency response. More over, there is a striking relationship between the induction temperature and the organism's environment. In different organisms the response is induced at very different temperatures. In each case, the organism would be expected to cope with such temperatures in its natural environment. Thus, in the fruit fly Drosophila melanogaster, induction occurs between 33-37 C, common temperatures on warm summer days (125). In thermophilic bacteria growing at 50 C, the proteins are induced when temperatures are raised to 60 C (51). In arctic fishes growing at ooe, they are induced at 5-lOoe (B. Maresca, personal communication). In mammals they are induced by fever tempera tures (122), and in soybeans they are induced in the field on hot sunny days (104).