HIGH LOCALIZATION OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE IN THE PYRENOIDS OF CHLAMYDOMONAS-REINHARDTII (CHLOROPHYTA), AS REVEALED BY CRYOFIXATION AND IMMUNOGOLD ELECTRON-MICROSCOPY/

The distribution of ribulose-1,5-bisphosphate carboxylase/oxygenase (R ubisco) in the chloroplasts of the unicellular green alga Chlamydomona s reinhardtii Dangeard was examined using cryotechnique and convention al fixation for immunogold electron microscopy. Both methods provided essentially identical results, although somewhat higher densities of g old particles indicating Rubisco molecules were recognized in the pyre noids of cryofixed cells. The gold particles were highly concentrated in the pyrenoid matrix within the chloroplasts. Even when considering the vast difference in volume between the pyrenoid and the rest of the chloroplast, more than 99% of the total Rubisco labeling in the chlor oplast was calculated to be present in the pyrenoid matrix. High local ization of Rubisco in the pyrenoid matrix was also recognized regardle ss of cell age, based on immunofluorescence microscopy of the same en bloc samples. These results are inconsistent with a recent immunocytoc hemical study employing cryotechnique in which more than 90% of the to tal Rubisco was recognized in the thylakoid region (thylakoid membrane s and stroma) of C. reinhardtii cells. Rubisco highly localized in the pyrenoid matrix may take part in active photosynthetic CO2 fixation a nd/or the CO2 concentrating mechanism.