Our recent molecular studies revealed two divergent PEP-carboxylase (PEPC [Ppc]) encoding genes in the green microalga Chlamydomonas reinhardtii, CrPpc1 and CrPpc2, which are coordinately responsive to changes in inorganic-N and -C supply at the transcript level [Mamedov, T.G., Moellering, E.R. and Chollet, R. (2005) Identification and expression analysis of two inorganic C- and N-responsive genes encoding novel and distinct molecular forms of eukaryotic phosphoenolpyruvate carboxylase in the green microalga C. reinhardtii, Plant J. 42, 832-843]. Here, we report the distribution of these two encoded catalytic subunits in the minor Class-1 and predominant Class-2 PEPC enzyme-forms, the latter of which is a novel high-molecular-mass, hetero-oligomeric complex containing both CrPpc1 (p109) and CrPpc2 (p131) polypeptides. The Class-1 enzyme, however, is a typical PEPC homotetramer comprised solely of p109. We also document that the amount of both CrPpc1/2 catalytic subunits is up-/down-regulated by varying levels of NH(4)(+) supplied to the culture medium.