Cloning and expression of the structural gene for beta-glucosidase of Kluyveromyces fragilis in Escherichia coli and Saccharomyces cerevisiae.

Cellobiose, the last product in cellulose degradation, is converted into two molecules of glucose by a beta-glucosidase. S. cerevisiae does posses the structural gene for a beta-glucosidase, but it is very poorly expressed; we thus decided to isolate and characterize that of Kluyveromyces fragilis. We constructed in E. coli HB101 strain a genomic library of the Kluyveromyces fragilis Y610 strain (ATCC 12424), a yeast able to grow on cellobiose and which constitutively produces the beta-glucosidase. The structural gene for beta-glucosidase was identified by its expression in E. coli. The initial isolated cosmid KF1 contained an insert of 35 Kb and by successive subcloning the insert size was reduced to 3.5 Kb (KF4). This cloned beta-glucosidase gene introduced in S. cerevisiae by transformation is expressed at a level of about 500 times that of K. fragilis. We checked by Southern hybridization that the high expression level was not due to a rearrangement of K. fragilis DNA during the cloning experiments. Nevertheless to obtain yeast transformants able to grow on cellobiose a yeast strain whose permeability to sugar is increased must be used and this last point is discussed.