Recent data have challenged the long held view that the product of nitrogenase activity, ammonia, is transferred from the symbiosome to the host cell cytosol where it is assimilated (reviewed in Day et al. 2001). Waters et al. (1998) have presentedevidence for the excretion of fixed N as alanine by bacteroids isolated from soybean (Glycine max [L.] Merr.) and purified by sucrose density gradient fractionation. However, excretion and accumulation of alanine by bacteroids was highest at pO_2 less than 0.01 while nitrogenase activity was maximal at pO_2 of 0.06. On the other hand, flow-chamber experiments under conditions where nitrogenase activity was optimized, also with isolated bacteroids from soybean, identified ammonia as the major excreted product of fixation (Li et al. 2001). A significant difference between the two types of experimental approach was the removal of the excreted products of fixation in the flow-through system while they accumulated in the closed system used by Waters et al.(1998) and Allaway et al. (2002) have suggested that this is the explanation for the conflicting results.
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