Pyrenoid localization of Rubisco in relation to the cell cycle and growth phase of Dunaliella tertiolecta (Chlorophyceae)

Abstract

Localization of ribulose 1,5-bisphosphate carboxylase/oxygenase in pyrenoids of the chlorophyte Dunaliella tertiolecta Butcher was analyzed using immunofluorescence. Some cells displayed distinct pyrenoid-Rubisco staining (i.e. Rubisco was highly concentrated in the pyrenoid), whereas others displayed weak staining throughout the chloroplast stroma. Paired pyrenoid-Rubisco units were observed, suggesting that pyrenoid division had occurred. The percentage of cells having distinct pyrenoid-Rubisco staining (PR-index) was depressed drastically by darkness, but the cell cycle, which was monitored by immunofluorescence of the proliferating cell nuclear antigen (PCNA), was not affected. Inversely, neither the cell cycle inhibitors hydroxyurea and colchicine nor short-term nutrient depletion affected the PR-index. These results, along with the observation that cells with or without distinct pyrenoid-Rubisco staining had similar DNA contents, suggest that variation in pyrenoid localization of Rubisco was not cell cycle dependent. In addition, the PR-index increased along with growth rates in the early exponential growth phase and decreased to zero in the late stationary phase. After a stationary culture was diluted by fresh medium, the PR-index increased from 0 to high levels within 24 hours. These results suggest that pyrenoid-localization of Rubisco is associated with the active growth phase.