The groEx cloned from endosymbiotic Gram-negative X-bacteria in the xD strain of Amoeba proteus has potent intragenic promoters within the coding region of groESx in addition to the heat-shock consensus promoters at 5`-extended region of the structural gene. When the gene was subcloned in plasmid vectors, GroESx and GroELx could be highly expressed separately at the temperature above 30℃ by these promoters. groE mutants complemented with the gene formed colony at nonpermissive temperature for the mutant and supported plaque formation for λ phage. Thus, the gene products were found to have chaperonin function in heterologous combination with groE gene products of E coli. In growth experiments, E. coli transformed with the gene grew to double density of control E. coli at 42 and 45℃. Accumulation of both GroESx and GroELx appeared to relieve early growth arrest of E. coli at these temperatures and enabled the cells to maintain viability over 50% at 47℃ for 8 h. Thus, large accumulation of GroES and GroEL analogues appears to promote the cell growth at 45℃ and to confer thermotolerance to E. coli at 47℃.