Onset of gluconate-H+ symport in Schizosaccharomyces pombe is regulated by the kinases Wis1 and Pka1, and requires the gti1+ gene product

In the fission yeast Schizosaccharomyces pombe, glucose represses onset of gluconate-H+ symport and inhibits transiently the activity of the symport protein. Wild-type cells harvested from high glucose medium take up gluconate very slowly and the rate of uptake is increased 150-fold in response to glucose starvation. Here it is shown that an intact cAMP cascade is necessary to prevent premature onset in the presence of high glucose concentrations. Cells which have lost either adenylate cyclase (Cyr1) or cAMP-dependent protein kinase (Pka1) transport gluconate up to 60-fold faster than wild-type cells when harvested from high glucose medium. Moreover, inactivation of the stress-sensing Wis1-Sty1 MAP kinase pathway, by loss of Wis1 MAP kinase kinase, diminishes 10-fold the onset of gluconate uptake in response to starvation. A mutant was identified showing a comparable phenotype. By complementation, the gti1+ (gluconate transport inducer 1) gene has been isolated. Disruption of gti1 reduces starvation-induced onset by a similar factor to that observed in wis1 delta cells. Cells over-expressing gti1+ induce gluconate uptake much faster resulting in a threefold higher uptake rate, although gti1+ does not code for the gluconate transport protein. In contrast to the repression of onset, transient downregulation of the gluconate symporter is independent of Pka1 activity and requires ongoing glucose influx. Addition of glucose to starved cyr1 delta cells reduces uptake 9-fold, whereas starved pka1 delta cells, which are able to synthesise cAMP, respond with a 60-fold decrease in transport.