Thermophilic organisms from each of the three phylogenetic domains (Bacteria, Archaea, and Eucarya) acquired thermotolerance after heat shock. Bacillus caldolyticus grown at 60°C and heat shocked at 69°C for 10 min showed thermotolerance at 74°C, Sulfolobus shibatae grown at 70°C and heat shocked at 88°C for 60 min showed thermotolerance at 95°C, and Thermomyces lanuginosus grown at 50T and heat shocked at 55°C for 60 min showed thermotolerance at 58°C. Determinations of protein synthesis during heat shock revealed differences in the dominant heat shock proteins for each species. For B. caldolytieus, a 70-kDa protein dominated while for S. shibatae, a 55-kDa protein dominated and for T. lanuginosus, 31-to 33-kDa proteins dominated. Reagents that disrupted normal protein synthesis during heat shock prevented the enhanced thermotolerance.Acquired thermotolerance refers to the enhanced survival of organisms at lethal temperatures after a brief exposure to near-lethal temperatures (2, 13). This response correlates with the synthesis of a small number of proteins known as heatshock proteins (HSPs), which has led to the hypothesis that thermotolerance depends on one or more of these specific proteins (10, 12, 19). There have been many investigations to test this hypothesis and to clarify the role of specific HSPs in thermotolerance (1,8,15). Two generalizations that may be made from the result of this research are that (i) HSPs do indeed play an important role in acquired thermotolerance and (ii) different species use different strategies (different combinations of HSPs and/or other macromolecules) in acquiring thermotolerance. Extending our knowledge of thermotolerance and the associated HSPs to other species may therefore provide additional insights into similarities and differences in these strategies. Most previously published research in this area has focused on mesophilic organisms (20,21,27), with only a few reports about thermophiles (7,25,28 (25). Finally, samples of conidia of T. lanuginosus (4) incubated for 4 h at 50°C with agitation in a modified YPS medium (T1 medium) (4), which sprouted over 90% of them, were heat shocked at 55°C for 60 min and challenged at 58°C. Viable conidia were enumerated by counting mycelial colonies that grew from dilutions on T1 medium solidified with 2% agar after 48 h at 450C.Under our experimental conditions, all three thermophilic species acquired thermotolerance, although their response times and the magnitudes of their tolerances varied (Fig. 1).The difference in survival of cells with and without heat shock was sufficiently large that despite variations between experiments, there was no doubt that some molecular adaptation had occurred. To determine whether this adaptation was associated with changes in protein synthesis like those observed in mesophilic species, we monitored changes in protein synthesis during the heat shock treatments of these thermophiles.Heat shock protein synthesis and gel electrophoresis. Protein synthesis at normal and heat shock temperatures w...