Molybdenum cofactor amounts in Chlamydomonas reinhardtii depend on the Nit5 gene function related to molybdate transport

Strain 21gr from Chlamydomonas reinhardtii is a cryptic mutant defective in the Nit5 gene related to the biosynthesis of molybdenum cofactor (MoCo). I n spite of this mutation, this strain has active MoCo and can grow on nitra te media. In genetic crosses, the Nit5 mutation cosegregated with a phenoty pe of resistance to high concentrations of molybdate and tungstate. Molybda te/tungstate toxicity was much higher in nitrate than in ammonium media. St rain 21gr showed lower amounts of MoCo activity than the wild type both whe n grown in nitrate and after growth in ammonium and nitrate induction. Howe ver, nitrate reductase (NR) specific activity was similar in wild type and 21gr cells. Tungstate, either at nanomolar concentrations in nitrate media or at micromolar concentrations during growth in ammonium and nitrate induc tion, strongly decreased MoCo and NR amounts in wild-type cells but had a s light effect in 21gr cells. Molybdate uptake activity of ammonium-grown cel ls from both the wild-type and 21gr strains was small and blocked by sulpha te 0.3 mM. However, cells from nitrate medium showed a molybdate uptake act ivity insensitive to sulphate. This uptake activity was much higher and mor e sensitive to inhibition by tungstate in the wild type than in strain 21gr . These results suggest that strain 21gr has a high affinity and low capaci ty molybdate transport system able to discriminate efficiently tungstate, a nd lacks a high capacity molybdate/tungstate transport system, which operat es in wild-type cells upon nitrate induction. This high capacity molybdate transport system would account for both the stimulating effect of molybdate on MoCo amounts and the toxic effects of tungstate and molybdate when pres ent at high concentrations.