MODE OF ACTION OF THE QCR9 AND CAT3 MUTATIONS IN RESTORING THE ABILITY OF SACCHAROMYCES-CEREVISIAE TPS1 MUTANTS TO GROW ON GLUCOSE

Mutations in the TPS1 gene, which encodes trehalose-6-P synthase, caus e a glucose-negative phenotype in Saccharomyces cerevisiae. Antimycin A or disruption of the QCR9 gene, which encodes one subunit of the cyt ochrome bc(1) complex, restore the ability to grow in glucose-containi ng media. Under these conditions the cell excreted a large amount of g lycerol, corresponding to about 20% of the glucose taken up. Suppressi on appears to be achieved by diversion of accumulated glycolytic inter mediates to the production of glycerol, thereby providing NAD(+) and p hosphate for the glyceraldehyde-3-P dehydrogenase reaction. Analysis o f the mutation sci1-1, which also suppresses the glucose-negative phen otype of tps1 mutants, showed that glucose transport was decreased in sci1-1 mutants. The gene SCI1 was cloned and its nucleotide sequence r evealed it to be identical to CAT3/SNF4. The suppression mediated by s cil-1 is attributable to a decrease in glycolytic flux.