Sugar repression in the methylotrophic yeast Hansenula polymorpha studied by using hexokinase-negative, glucokinase-negative and double kinase-negative mutants.

Two glucose-phosphorylating enzymes, a hexokinase phosphorylating both glucose and fructose, and a glucose-specific glucokinase were electrophoretically separated in the methylotrophic yeast Hansenula polymorpha. Hexokinase-negative, glucokinase-negative and double kinase-negative mutants were isolated in H. polymorpha by using mutagenesis, selection and genetic crosses. Regulation of synthesis of the sugar-repressed alcohol oxidase, catalase and maltase was studied in different hexose kinase mutants. In the wild type and in mutants possessing either hexokinase or glucokinase, glucose repressed the synthesis of maltase, alcohol oxidase and catalase. Glucose repression of alcohol oxidase and catalase was abolished in mutants lacking both glucose-phosphorylating enzymes (i.e. in double kinase-negative mutants). Thus, glucose repression in H. polymorpha cells requires a glucose-phosphorylating enzyme, either hexokinase or glucokinase. The presence of fructose-phosphorylating hexokinase in the cell was specifically needed for fructose repression of alcohol oxidase, catalase and maltase. Hence, glucose or fructose has to be phosphorylated in order to cause repression of the synthesis of these enzymes in H. polymorpha suggesting that sugar repression in this yeast therefore relies on the catalytic activity of hexose kinases.