Abstract
The photosynthetic nature of the initial stages of nitrate assimilation, namely, uptake and reduction of nitrate, has been investigated in cells of the cyanobacterium Anacystis nidulans treated with l-methionine dl-sulfoximine to prevent further assimilation of the ammonium resulting from nitrate reduction. The light-driven utilization of nitrate or nitrite by these cells results in ammonium release and is associated with concomitant oxygen evolution. Stoichiometry values of about 2 mol oxygen evolved per mol nitrate reduced to ammonium and 1.5 mol oxygen per mol nitrite have been determined in the presence of CO 2, as well as in its absence, with nitrate or nitrite as the only Hill reagent. This indicates that in A. nidulans water photolysis directly provides, without the need for carbon metabolites, the reducing power required for the in vivo reduction of nitrate and nitrite to ammonium, processes which are besides strongly inhibited when the operation of the photosynthetic noncyclic electron flow is blocked. Evidence indicating the participation of concentrative transport system(s) in the uptake of nitrate and nitrite by A. nidulans is also presented. The operation of these energy-requiring systems seems to account for the sensitivity to ATP-synthesis inhibitors exhibited by nitrate and nitrite utilization in l-methionine dl-sulfoximine-treated cells. The utilization of nitrate by A. nidulans cells, concomitant with oxygen evolution, can therefore be considered as a genuinely CO 2-independent photosynthetic process that makes direct use of photosynthetically generated assimilatory power.
Published Version
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