Molecular cloning, DNA structure and expression of the Escherichia coli D‐xylose isomerase

The D‐xylose isomerase (EC 5.3.1.5) gene from Escherichia coli was cloned and isolated by complementation of an isomerase‐deficient E. coli strain. The insert containing the gene was restriction mapped and further subcloning located the gene in a 1.6‐kb Bg/II fragment. This fragment was sequenced by the chain termination method, and showed the gene to be 1002 bp in size. The Bg/II fragment was cloned into a yeast expression vector utilising the CYCl yeast promoter. This construct allowed expression in E. coli grown on xylose but not glucose suggesting that the yeast promoter is responding to the E. coli catabolite repression system. No expression was detected in yeast from this construct and this is discussed in terms of the upstream region in the E. coli insert with suggestions of how improved constructs may permit achievement of the goal of a xylose‐fermenting yeast.