An inverted repeat corresponding to parts of the centrin gene of Chlamydomonas reinhardtii was placed downstream of the NIT1 promoter, which is induced by ammonium starvation. After induction, transformants developed centrin deficiency as assayed by immunofluorescence, Western blotting, and Northern blotting. The effect was reversible, demonstrating that the NIT1 promoter allowed controlled RNA interference in Chlamydomonas reinhardtii.