Rubisco of Dunaliella tertiolecta is redistributed between the pyrenoid and the stroma as a light/shade response

We used an immunofluorescence technique to investigate the effects of varying light regimes on ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in individual cells of the chlorophyte Dunaliella tertiolecta Butcher. These studies were carried out between September 1995 and February 1996. The population was heterogeneous with respect to Rubisco localization in the pyrenoid, i.e. in some cells Rubisco staining was highly concentrated in the pyrenoid while in others it was evenly distributed throughout the chloroplast stroma. When light intensity was varied sevenfold, the fraction of the cell population that displayed distinct Rubisco staining in the pyrenoid (PR-index) was correlated with light intensity, although the average Rubisco abundance per cell or per total cellular protein appeared fairly constant. In darkness, or when treated with 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU), the PR-index decreased markedly during the first 4 and 3 h, respectively, and then remained at low levels, while the cell division cycle progression remained unaffected. We conclude that D. tertiolecta probably possesses an adaptive mechanism, i.e. the redistribution of Rubisco between the pyrenoid (probably the active site of Rubisco activation and CO2 fixation) and the stroma (probably a reservoir of deactivated Rubisco that is readily available for transport to the pyrenoid and activation), to respond to variations in irradiance or photosynthetic inhibition. Our results also suggest that this mechanism is insensitive to slight variations in growth irradiance and to seasonal changes in photoperiod and temperature.