Fd:nitrite oxidoreductases (EC 1.7.7.1, hereafter referred to as nitrite reductase) catalyze the six-electron reduction of nitrite to ammonia, using reduced Fd as the electron donor (Knaff, 1996). Fd-dependent nitrite reductases have been isolated and characterized from a number of higher plants, algae, and cyanobacteria and have been shown to be monomeric proteins with molecular masses near 63 kD, which contain a single [4Fe-4S] cluster and a single siroheme (which serves as the binding site for nitrite) as prosthetic groups (Knaff, 1996). The spinach (Spinacea oleracea) leaf enzyme, which is localized in the chloroplast stromal space, has been particularly well characterized (Knaff, 1996), and its complete amino acid sequence has been deduced from a cDNA sequence and N-terminal amino acid sequencing of the mature form of the protein (Back et al
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