The Streptomyces rubiginosus xylA gene was cloned and expressed in Saccharomyces cerevisiae. No xylose isomerase activity could be detected. The produced xylose isomerase protein was insoluble and could only be recovered from cell lysates by extraction with the detergent sodium dodecyl-sulfate. In contrast, expression of the xylA gene from Thermus thermophilus in the same host strain resulted in soluble xylose isomerase protein with activities of I U mg(-1) protein. Comparison of available 3D models suggests that the higher number of intra-subunit ion-bridges in the Thermus thermophilus xylose isomerase may stabilise the protein structure and promote folding by Saccharomyces cerevisiae.