The inorganic carbon requirements for nitrogen assimilation

In the green alga Selenastrum minutum (Naeg.) Collins the assimilation of NH 4 + into the full suite of protein amino acids requires at least three separate and distinct inorganic carbon fixing reactions, catalyzed by the enzymes ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), phosphoenolpyruvate carboxylase (PEPC), and carbamoyl phosphate synthetase. In this paper we examine the requirements for CO 2 fixation of NH 4 + assimilation in this organism. When grown under N-sufficient conditions, NH 4 + assimilation is directly dependent upon photosynthetic CO 2 fixation to provide carbon skeletons for amino acid synthesis. When cultured under N-limited conditions, the cells accumulate starch, which is then available for amino acid synthesis. This alleviates the requirement of photosynthetic CO 2 fixation for NH 4 + assimilation. N-limited cells, however, still exhibit a nonphotosynthetic CO 2 requirement for N assimilation that is mediated through PEPC. This activity of PEPC increases during N assimilation to replenish TCA cycle intermediates consumed during amino acid synthesis. The in vivo activity of this enzyme is tightly regulated so that there are ~0.3 moles C fixed per mole N assimilated. In S. minutum PEPC is regulated primarily by the ratio of glutamine/glutamate, thus providing a mechanism by which primary NH 4 + assimilation modulates the supply of carbon for amino acid biosynthesis. Activation of PEPC during NH 4 + assimilation occurs in both the light and the dark.