The Intracellular Localization of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase in Chlamydomonas reinhardtii

The pyrenoid is a proteinaceous structure found in the chloroplast of most unicellular algae. Various studies indicate that ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is present in the pyrenoid, although the fraction of Rubisco localized there remains controversial. Estimates of the amount of Rubisco in the pyrenoid of Chlamydomonas reinhardtii range from 5% to nearly 100%. Using immunolocalization, the amount of Rubisco localized to the pyrenoid or to the chloroplast stroma was estimated for C. reinhardtii cells grown under different conditions. It was observed that the amount of Rubisco in the pyrenoid varied with growth condition; about 40% was in the pyrenoid when the cells were grown under elevated CO₂ and about 90% with ambient CO₂. In addition, it is likely that pyrenoidal Rubisco is active in CO₂ fixation because in vitro activity measurements showed that most of the Rubisco must be active to account for CO₂-fixation rates observed in whole cells. These results are consistent with the idea that the pyrenoid is the site of CO₂ fixation in C. reinhardtii and other unicellular algae containing CO₂-concentrating mechanisms.