Cloning and analysis of the tomato nitrate reductase-encoding gene: protein domain structure and amino acid homologies in higher plants.

We have cloned and sequenced the nitrate reductase (NR)-encoding gene (nia) from tomato. When compared to the two Nicotiana tabacum nia structural genes, this 5-kb tomato gene shows a highly conserved structure, the coding sequence being interspersed with three introns at the same positions. Nucleotide sequences of the 5' promoter regions are not homologous, except for a 250-bp fragment. This small region might be involved in the similar regulation of the nia expression in tomato and tobacco plant species. The tomato gene codes for a 911 amino acid (aa) polypeptide chain. This sequence was aligned with and compared to other higher plant NR sequences. This alignment clearly identifies the three catalytic domains of NR, namely, a molybdopterin cofactor-binding domain, a heme domain and a FAD/NADH domain. On the other hand, it suggests that the less conserved 80-aa N-terminal region, containing a striking acidic aa cluster, is an additional domain bearing regulatory or structural function.