Lipid Biosynthesis and its Coordination with Cell Cycle Progression
Kwok, Alvin C. M.
The activation of cell cycle regulators at the G₁/S boundary has been linked to the cellular protein synthesis rate. It is conceivable that regulatory mechanisms are required to allow cells to coordinate the synthesis of other macromolecules with cell cycle progression. The availability of highly synchronized cells and flow cytometric methods facilitates investigation of the dynamics of lipid synthesis in the entire cell cycle of the heterotrophic dinoflagellate Crypthecodinium cohnii. Flow cytograms of Nile red-stained cells revealed a stepwise increase in the polar lipid content and a continuous increase in neutral lipid content in the dinoflagellate cell cycle. A cell cycle delay at early G₁, but not G₂/M, was observed upon inhibition of lipid synthesis. However, lipid synthesis continued during cell cycle arrest at the G₁/S transition. A cell cycle delay was not observed when inhibitors of cellulose synthesis and fatty acid synthesis were added after the late G₁ phase of the cell cycle. This implicates a commitment point that monitors the synthesis of fatty acids at the late G₁ phase of the dinoflagellate cell cycle. Reduction of the glucose concentration in the medium down-regulated the G₁ cell size with a concomitant forward shift of the commitment point. Inhibition of lipid synthesis up-regulated cellulose synthesis and resulted in an increase in cellulosic contents, while an inhibition of cellulose synthesis had no effects on lipid synthesis. Fatty acid synthesis and cellulose synthesis are apparently coupled to the cell cycle via independent pathways.
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