Abstract
A mutant of Saccharomyces cerevisiae that was selected for resistance to carbon catabolite repression also had reduced hexokinase activity. Hexokinase isoenzymes were purified from mutant and wild type cells. The specific glucokinase and hexokinase isozyme PI were present at normal levels in mutant and wild type, but no hexokinase isozyme PII activity was detected in the mutant. Staining for enzyme activity after electrophoresis of crude extracts also indicated that hexokinase PII was absent in the mutant. Mutant and wild type segregants gained by tetrad analysis were investigated electrophoretically. Staining for enzyme activity confirmed that catalytically inactive hexokinase PII and the defect in carbon catabolite repression always co-segregated. The results support the hypothesis that hexokinase PII might mediate carbon catabolite repression.
Highlights
A mutant of Saccharomyces cerevisiae that was se- the maltose uptake system was disregulated in hex2 mutants, lectedforresistance to carboncataboliterepression which ledto a strong inhibition of cell metabolism by maltose had reduced hexokinase activity
Sistant tocarbon catabolite repression was described by Zimmermann and Scheel (1977).Three mutant classes wereidentified (Zimmerman and Scheel, 1977; Entian and Zimmermann, 1980). (a)hex2 mutants were no longer repressible by glucosefor invertase, maltase, malate dehydrogenase, and respiratory enzymes.Hexokinase activity was strongly decreased (Entian et al, 1977). ( b )hex2 mutants had a similar a corresponds to the rnatlng type, andhls 4 causes nutrltlonal requlrement f o r hlstldlne
The results described here show that the decreased hexokinase activity in hexl mutants is attributable to anabsent or catalytically inactive hexokinase PII
Summary
A mutant of Saccharomyces cerevisiae that was se- the maltose uptake system was disregulated in hex mutants, lectedforresistance to carboncataboliterepression which ledto a strong inhibition of cell metabolism by maltose had reduced hexokinase activity. In the yeast Saccharomyces cerevisiae enormous differences in the activities of certain enzymes have been reported between cellsgrowingonglucose and on nonfermentable carbon sources, such as ethanol or acetate Such differences were observedfor some enzymesof the tricarboxylicacid cycle and glyoxylate cycle (Polakis and Bartley, 1965), respiratory enzymes (Polakis et al, 1965), gluconeogenic enzymes(Gancedo et al, 1965;Witt et al, 1965;Gancedo and Schwerzmann, 1976),cy-glucosidases (Wijk and van Ouwehand, 1969), and invertase (P-fructofuranosidase) (Gascon et al, 1968). This phenomenon has been called carbon catabolite repression in can phosphorylate glucose,fructose, and mannose. (a)hex mutants were no longer repressible by glucosefor invertase, maltase, malate dehydrogenase, and respiratory enzymes.Hexokinase activity was strongly decreased (Entian et al, 1977). M A ~ 2 - a ccauses largely constltutlve, but glucose repressible synthesis of maltase (Zlmmermann a n d Eaton. 1974) M A L J allows f o r an lnduced synthesls of maltasc, whereas S u c J is the structural gene of lnvcrtase IGrossmann and Zimermann. 19791. c a t 2 - I allows rapld derepresslon of various enzymes after qrowth on glucose(zimmerrnann et al.. 1977). i l e x 1 causes non-repressible maltase, invertase and malate dehydrogenase
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