Regulation of the nitrate reductase level in anacystis nidulans: activity decay under nitrogen stress

The in vivo stability of ferredoxin-nitrate reductase from the cyanobacterium Anacystis nidulans under conditions of inhibited protein synthesis has been studied in nitrate-grown cells. A light-promoted rapid decay in cellular nitrate reductase activity took place in the absence of any added nitrogen source, but not in the presence of nitrate, nitrite, or ammonium. The inactivation process seemed to proceed in two sequential steps. The first step required both light and oxygen, and was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) or, to a lesser extent, by sulfhydryl-containing compounds. The resulting inactive form of nitrate reductase, apparently suffering from an oxidative modification, could be reactivated in vivo either by switching-off the light or by addition of inorganic nitrogenous compounds. Prolonged illumination of the cells in the absence of a nitrogen source led to further modification of the enzyme, which could not be reversed. Stability of the active enzyme appears to be a decisive factor contributing to the determination of the actual level of nitrate reductase in A. nidulans cells.