Iron uptake by the yeast Pichia guilliermondii. Flavinogenesis and reductive iron assimilation are co-regulated processes.

Pichia guilliermondii cells overproduce riboflavin (vitamin B2) in responce to iron deprivation. The increase in ferrireductase activity in iron-starved P. guilliermondii cells correlated with the increase in flavin excretion. As in Saccharomyces cerevisiae, a typical b-type cytochrome spectrum was associated with the plasma membrane fraction of P. guilliermondii and the cell ferrireductase activity was strongly inhibited by diphenylene-iodonium, an inhibitor of flavoproteins, in both yeasts. Mutants of P. guilliermondii with increased ferrireductase activity were selected for further investigation of the relationship between iron reduction/uptake and flavin production. The obtained mutation has been called hit (high iron transport). A hit mutant with a single recessive mutation showed the following phenotype: high ferrireductase activity, increased rate of iron uptake and elevated flavinogenic activity. Cu(II) (50 microns) strongly inhibited the growth of the hit mutant compared to the wild-type. The mutant cells grown in copper-supplemented medium (5-25 microns) showed an increase of the ferrireductase activity (up to 2-3 fold). The copper content of the mutant cells grown under these conditions was also higher (1.5-2 fold) than that of the wild-type. The role of the HIT gene of P. guillermondii in the regulation of iron, copper and flavin metabolisms is discussed.