The groESLx operon cloned from endosymbiotic Gram-negative X-bacteria in the xD strain of Amoeba proteus has heat-shock consensus promoters at an extended region of 5′ of the structural gene and specific promoters within the coding region of groESx. When the genes were subcloned in plasmid vectors, groESx and groELx could be highly expressed separately at temperatures above 30°C by heat-shock consensus promoters and the specific promoters, respectively. E. coli groE mutants complementing these genes formed colonies at non-permissive temperatures for the mutants and supported plaque formation for λ phage. Thus, the gene products were found to have chaperonin functions in heterologous combination with groE gene products of E. coli in vivo. In growth experiments, E. coli groE mutants transformed with groEx grew to a density twice that of E. coli groE mutants at 42 and 45°C. Accumulation of both GroESx and GroELx enabled the E. coli to maintain over 70% viability at 50°C for 6 hrs. Thus, accumulation of large amounts of GroESLx in E. coli appeared to promote cell growth and to confer thermotolerance at sublethal temperatures.