THE ASPERGILLUS-NIGER-NIAD GENE ENCODING NITRATE REDUCTASE - UPSTREAM NUCLEOTIDE AND AMINO-ACID-SEQUENCE COMPARISONS

The Aspergillus niger niaD gene has been sequenced and the inferred nitrate reductase (NR) protein found to consist of 867 amino acid residues (97 kDa). The gene is interrupted by six small introns, as deduced by comparison with the niaD gene of Aspergillus nidulans. The positions of these putative introns are conserved between the two fungi, although the sequences are dissimilar. The niiA gene, encoding nitrite reductase, the second reductive step in the nitrate assimilation pathway, is tightly linked to niaD and divergently transcribed in A. niger, similar to the general organisation in the related fungi, Aspergillus oryzae and A. nidulans. The nucleotide (nt) sequences of the intergenic region between niiA and NiaD (excluding the ATG translation start codon) of A. niger (1668 nt) and A. oryzae (1575 nt) were determined and compared with the previously determined A. nidulans (1262 nt) sequence. No striking extended nt regions of homology are observed in spite of the fact that transgenic strains with fungal niaD or the two control genes required for induction and repression show virtually normal regulation. Fungal NR shows considerable aa homology with higher plant NR, particularly within the co-factor domains for flavin adenoside dinucleotide, heme and molybdopterin cofactor.