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Polypeptide composition and enzyme activities of the pyrenoid and its regulation by CO 2 concentration in unicellular green algae

1991; Canadian Science Publishing; Volume: 69; Issue: 5 Linguagem: Inglês

10.1139/b91-136

1480-3305

Kazuyuki Kuchitsu, Mikio Tsuzuki, Shigetoh Miyachi,

Marine and coastal ecosystems

Effects of environmental conditions on the pyrenoid were investigated in unicellular green algae. During adaptation to CO 2 limitation, the pyrenoid and pyrenoid starch developed within several hours, while stroma starch was degraded, suggesting that metabolism around the pyrenoid is regulated independently from that in other stromal spaces. This pyrenoid development was light-dependent and inhibited by DCMU but was not affected by changes in nitrogen assimilation. Pyrenoids isolated from Chlamydomonas reinhardtii mainly consisted of the two subunits of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), and several other polypeptides were detected. Phosphoribulokinase was not localized in the pyrenoid. Isolated chloroplasts were fractionated to separate intact pyrenoids from stroma and membranes. Rubisco activity was detected in both, and about 70% was distributed in the pyrenoid fraction in low-CO 2 cells. The Rubisco content determined by SDS–PAGE, immunoblotting, and immunoelectron microscopy supported the above ratio. By fractionation of isolated chloroplasts under high ionic strength, a pyrenoid-like structure with high Rubisco activity was collected. It is suggested that the main carboxylation site in high-CO 2 cells is the stroma, whereas Rubisco is concentrated in the pyrenoid and possibly functions in CO 2 fixation under low-CO 2 conditions. Key words: Chlamydomonas reinhardtii, chloroplast, CO 2 concentration, photosynthetic CO 2 fixation, pyrenoid, ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco).