DNA sequencing in the phage λJA13isolated from a λEMBL3 Hansenula polymorphagenomic DNA library containing the nitrate reductase- (YNR1) and nitrite reductase- (YNI1) encoding genes revealed an open reading frame (YNT1) of 1524 nucleotides encoding a putative protein of 508 amino acids with great similarity to the nitrate transporters from Aspergillus nidulansand Chlamydomonas reinhardtii. Disruption of the chromosomal YNT1copy resulted in incapacity to grow in nitrate and a significant reduction in rate of nitrate uptake. The disrupted strain is still sensitive to chlorate, and, in the presence of 0.1 mM nitrate, the expression of YNR1 and YNI1 and the activity of nitrate reductase and nitrite reductase are significantly reduced compared with the wild-type. Northern-blot analysis showed that YNT1 is expressed when the yeast is grown in nitrate and nitrite but not in ammonium solution.

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