A gene encoding the rice (Oryza sativa L.) 90-kDa heat shock protein (OsHsp90) was introduced into Escherichia coli using the pGEX-6p-3 expression vector with a glutathione-S-transferase (GST) tag to analyze the possible function of this protein under heat stress for the first time. We compared the survivability of E. coli (BL21) cells transformed with a recombinant plasmid containing GST-OsHsp90 fusion protein with control E. coli cells transformed with the plasmid containing GST and the wild type BL21 under heat shock after isopropyl beta-D: -thiogalactopyranoside induction. Cells expressing GST-OsHsp90 demonstrated thermotolerance at 42, 50, and 70 degrees C, treatments that were more harmful to cells expressing GST and the wild type. Further studies were carried out to analyze the heat-induced characteristics of OsHsp90 at 42, 50, and 70 degrees C in vitro. When cell lysates from E. coli transformants were heated at these heat stresses, expressed GST-OsHsp90 prevented the denaturation of bacterial proteins treated with 42 degrees C heat shocks, and partially prevented that of proteins treated at 50 and 70 degrees C; meanwhile, cells expressing GST-OsHsp90 withstood the duration at 50 degrees C. These results indicate that OsHsp90 functioned as a chaperone, binding to a subset of substrates, and maintained E. coli growth well at high temperatures.