The groE homologous genes of the anaerobic thermophile Thermoanaerobacter sp. Rt8.G4 (TRt) have been isolated, sequenced and analysed. The TRt groES and groEL encode subunits of chaperonin 10 (Cpn10) and chaperonin 60 (Cpn60) of 94 and 541 amino acids, respectively, and are arranged in that order forming the open reading frames (ORFs) of a bicistronic operon. A controlling inverted repeat of chaperone expression (CIRCE) element lies between the consensus promoter of the operon and TRt groES. At optimum growth temperature (65 degreesC) the chaperonins of TRt are expressed, but production of Cpn60 increases significantly following temperature increases of 3-10 degreesC. Functionally intact recombinant TRt chaperonins were produced in Escherichia coli. However, owing to codon incompatibility, replacement of consecutive AGA codons in the gene encoding TRt Cpn60 was necessary for optimum expression in this heterologous host.