Molecular characterization of tobacco sulfite reductase: enzyme purification, gene cloning, and gene expression analysis.

A cDNA clone, NtSiR1, that encodes the precursor of ferredoxin-dependent sulfite reductase (Fd-SiR) has been isolated from a cDNA library of tobacco (Nicotiana tabacum cv. SR1). The identity of the cDNA was established by comparison of the purified protein and the predicted structure with the nucleotide sequence. The amino terminus of the purified enzyme was Thr62 of the precursor protein, and the mature region of NtSiR1 consisted of 632 amino acids. Tobacco Fd-SiR is 82, 77, and 48% identical with Fd-SiRs from Zea mays, Arabidopsis thaliana, and a cyanobacterium, respectively. Significant similarity was also found with Escherichia coli NADPH-SiR in the region involved in ligation of siroheme and the [4Fe-4S] cluster. On Northern blot analysis, a transcript of NtSiR1 was detected in leaves, stems, roots, and petals in similar amounts. We also isolated a genomic SiR clone named gNtSiR1. It consists of 8 exons and 7 introns. Genomic Southern blot analysis indicated that at least two SiR genes are present in the tobacco genome.