Relationship of nitrite and hydroxylamine reductases to nitrate assimilation and nitrogen fixation in Azotobacter agile.

The nitrogen fixation process has been recently reviewed by Wilson (1952), Burris (1956), Shug et al. (1956), and Gest et al. (1956). Although the characteristics of the over-all process in vivo are well known, neither the pathway nor the enzymatic steps has been established. On the other hand, a number of enzymes from bacteria, fungi and higher plants have been characterized which together can catalyze the reduction of nitrate to ammonia by way of nitrite and hydroxylamine. The properties of these systems and their possible role in the main pathway of nitrate assimilation have also been recently reviewed (Verhoeven, 1956; Taniguchi et al., 1956; Nason, 1956; McElroy and Spencer, 1956). That nitrogen fixation and nitrate assimilation might have a common pathway has been suggested by a number of workers. The present paper reports the presence in Azotobacter agile (A. vinelandii) of pyridine nucleotide nitrite and hydroxylamine reductases with properties similar to those already described from Neurospora and soybean leaves (Nason et al., 1954; Zucker and Nason, 1955). These systems have been characterized, and some indication of their physiological significance in nitrogen fixation and nitrate assimilation has been obtained from adaptation experiments.