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. guillermondii 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 m) strongly inhibited the growth of the hit mutant compared to the wild-type. The mutant cells grown in copper-supplemented medium (5–25 m) 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.