COMPOSITION AND FUNCTION OF PYRENOIDS - CYTOCHEMICAL AND IMMUNOCYTOCHEMICAL APPROACHES

At present, little physiological or biochemical data exist for pyrenoids mainly because isolation of intact pyrenoids using standard cell-fractionation methodology has met with only limited success. Techniques of microscopical cytochemistry and immunocytochemistry, however, readily lend themselves to the in situ investigation of pyrenoid composition. Immunocytochemical analyses have demonstrated that in evolutionarily diverse groups of pyrenoid-containing algae and hornworts, the Calvin cycle enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) is predominantly pyrenoid-localized. Moreover, the localization of Rubisco activase to pyrenoids of green algae and hornworts indicates that pyrenoid-localized Rubisco is catalytically competent. Although pyrenoids are reported to contain polypeptides other than Rubisco and Rubisco activase, none have been identified with certainty. The exclusion of phosphoribulokinase from the pyrenoids of red and green algae indicates that pyrenoids do not possess the full complement of Calvin cycle enzymes. There have been reports that nitrate reductase is pyrenoid-localized in green algae; however, this remains a contentious issue. Why Rubisco is localized to the pyrenoid is not clear. Available evidence does not support the extension to pyrenoids of a model recently devised for cyanobacterial carboxysomes in which the carboxysome is identified as an integral component of the inorganic carbon concentrating mechanism. Instead, perhaps the pyrenoid represents an evolutionary intermediate between cyanobacterial carboxysomes and the condition in which Rubisco is distributed throughout the chloroplast stroma.