A 6-kb DNA fragment from an extreme thermophile, Thermus thermophilus, carrying the genes for cytochrome oxidase ba3 subunit I (cbaA) and the ribosomal protein S15 (rpsO) was cloned into Escherichia coli. The gene rpsO was sequenced. The deduced amino acid sequence exhibits 59% identity to the corresponding protein from E. coli. Expression of rpsO in E. coli requires the use of a fully repressed inducible promoter because S15 from T. thermophilus is toxic for E. coli cells. When purified without denaturation from either overproducing E. coli strain or from T. thermophilus ribosomes, the S15 protein is stable and binds a cloned T. thermophilus 16S rRNA fragment (nucleotides 559-753), with low identical dissociation constants (2.5 nM), thus demonstrating that the thermophilic protein folds correctly in a mesophilic bacterium. The rRNA fragment bound corresponds in position and structure to the 16S rRNA fragment of E. coli. A similar high affinity was also found for the binding of S15 from T. thermophilus or E. coli to the corresponding E. coli 16S rRNA fragment, whereas a slightly lower affinity was observed in binding experiments between E. coli S15 and T. thermophilus 16S rRNA fragment. These results suggest that S15 from T. thermophilus recognizes similar determinants in both rRNA fragments. Competition experiments support this conclusion.