Multiplicity of Hydroxylamine Reductase Activities in Neurospora Crassa.

Enzymes capable of catalyzing the hydroxylamine-dependent. oxidation of pyridine nucleotides have been studied in Salmonella typhimurium (1, 2), Escherichia coli (3-5), Bacillus subtilis (6, 7), Azotobacter agile (8, 9), Neurospora crassa (lo), and soybean leaves (11). Siegel, Click, and Monty (1) and Siegel and Monty (2) demonstrated that reduced triphosphopyridine nucleotide-specific hydroxylamine reductase activity found in soluble extracts of S. typhimurium is entirely identified with the enzyme sulfite reductase (H&NADP oxidoreductase, EC 1.8.1.2). Thus, the level of hydroxylamine reductase varies in a coordinate fashion with the level of sulfite reduct’ase when different sulfur sources are utilized for growth of the bacteria, both activities being completely repressed on cysteinecontaining media. The integrity of six cistrons is required for maintenance of the ability to reduce sulfite to sulfide in Salmonella (12); a loss of function in any one of these cistrons leads to loss of all TPNH-hydroxylamine reductase activit,y in extracts (1). Competition experiments have implicated a common enzymatic site for the reduction of both sulfite and hydroxylamine (2). A constant ratio of activities with the two substrates was obtained throughout a partial purification of Salmonella sulfite reductase.l Mager (3), Lazzarini and Atkinson (4), and Kemp, Atkinson, Ehret, and Lazzarini (5) have presented evidence which strongly indicates the identity of the hydroxylamine and sulfite reductase activities of E. coli. In contrast to the results in bacteria, in which organisms hydroxylamine reduction appears to be an incidental property of an enzyme concerned with sulfur metabolism, Zucker and Nason (10) have described a pyridine nucleotide-dependent hydroxylamine reductase from N. cra.ssa whose level is “adaptive” to the presence of nitrate in the growth medium. The enzyme was active with either DPNH or TPNH and was inhibited by sulfite. The possible connection of this enzyme with the pathway of nitrate assimilation in Neurospora was cast in doubt, however, when Zucker and Nason (10) demonstrated the presence of this activity in a series of mutants (13) unable to utilize either nitrate or nitrite for growth. The nature of Neurospora hydroxylamine reduction and its relation to the sulfite reductase previously described in that organism by Leinweber and Monty (14) is explored in the pres-