The assimilation of NH4 causes a rapid increase in respiration to provided carbon skeletons for amino acid synthesis. In this study we propose a model for the regulation of carbon partitioning from starch to respiration and N assimilation in the green alga Selenastrum minutum. We provide evidence for both a cytosolic and plastidic fructose-1,6-bisphosphatase. The cytosolic form is inhibited by AMP and fructose-1,6-bisphosphate and the plastidic form is inhibited by phosphate. There is only one ATP dependent phosphofructokinase which, based on immunological cross reactivity, has been identified as being localized in the plastid. It is inhibited by phosphoenolpyruvate and activated by phosphate. No pyrophosphate dependent phosphofructokinase was found. The initiation of dark ammonium assimilation resulted in a transient increase in ADP which releases pyruvate kinase from adenylate control. This activation of pyruvate kinase causes a rapid 80% drop in phosphoenolpyruvate and a 2.7-fold increase in pyruvate. The pyruvate kinase mediated decrease in phosphoonolpyruvate correlates with the activation of the ATP dependent phosphofructokinase increasing carbon flow through the upper half of glycolysis. This increased the concentration of triosephosphate and provided substrate for pyruvate kinase. It is suggested that this increase in triosephosphate coupled with the glutamine synthetase mediated decline in glutamate, serves to maintain pyruvate kinase activation once ADP levels recover. The initiation of NH4 assimilation causes a transient 60% increase in fructose-2,6-bisphosphate. Given the sensitivity of the cytosolic fructose-1,6-bisphosphatase to this regulator, its increase would serve to inhibit cytosolic gluconeogenesis and direct the triosephosphate exported from the plastid down glycolysis to amino acid biosynthesis.Over the last decade there has been a significant increase in our knowledge of the control of carbon partitioning from starch to sucrose in photosynthetic tissue (34). In the proposed models, most of the carbon exported from the chloroplast is TP (see Table I for complete list of abbreviations). In the cytosol, Fru-2,6-P2 plays an important role in regulating the conversion of TP to sucrose through its effects on cytosolic It is well established that nitrogen exerts a major influence on respiratory carbon metabolism (1,9,15,16,31,(35)(36)(37)41). The assimilation of NH4' enhances starch breakdown and the flow of carbon into TCA cycle intermediates which are then used in amino acid biosynthesis. At present there is no model which provides an integrative picture of the mechanism by which NH4' assimilation regulates this carbon partitioning. In the present study we report changes in cellular metabolites during the dark assimilation of NH4' by the Nlimited green alga Selenastrum minutum. We also report preliminary data on the localization and regulation of PFK, PFP, and FBPase in this organism. Together, these results allow us to propose an integrative model for the control of carbon partitionin...