Dimers and oligomers of F-type ATP synthases have been observed previously in mitochondria of various organisms and for the CF o F 1 ATP synthase of chloroplasts of Chlamydomonas reinhardtii. In contrast to mitochondria, however, dimers of chloroplast ATP synthases dissociate at elevated phosphate concentration. This suggests a regulation by cell physiological processes.Stable isotope labeling of living cells and blue-native PAGE have been employed to quantitate changes in the ratio of monomeric to dimeric CF o F 1 ATP synthase. Chlamydomonas reinhardtii cells were cultivated photoautotrophically in the presence of 15 N and photomixotrophically at natural 14 N abundance, respectively.As compared to photoautotrophic growth, an increased assembly of ATP synthase dimers on the expense of preexisting monomers during photomixotrophic growth was observed, demonstrating a metabolic control of the dimerization process.