J.-D. Rochaix

Photosynthetic organisms adapt to changes in light quality by redistributing light excitation energy between two photosystems through state transition. This reorganization of antenna systems leads to an enhanced photosynthetic yield. Using a genetic approach in Chlamydomonas reinhardtii to dissect the signal transduction pathway of state transition, we identified a chloroplast thylakoid-associated serine-threonine protein kinase, Stt7, that has homologs in land plants. Stt7 is required for the phosphorylation of the major light-harvesting protein (LHCII) and for state transition.

The green unicellular alga Chlamydomonas reinhardtii has long been used as a model system for studying photosynthesis, chloroplast biogenesis, and flagellar function and assembly because of its well-defined genetics. The value of this organism has been greatly increased recently by the development of efficient methods for nuclear and chloroplast transformation. While homologous recombination appears to occur at a low frequency in the nuclear genome, random integrations can be exploited to tag genes of interest by insertional mutagenesis. Cloning of the nuclear genes by genomic complementation is also possible. Chloroplast genetic engineering has provided new and important insights into the molecular mechanisms of chloroplast gene expression and into the function of chloroplast proteins. C. reinhardtii is probably the best organism in which to perform chloroplast DNA surgery because of the ease of chloroplast transformation in this alga. C. reinhardtii might be described as the photosynthetic yeast. However, it offers additional new promising areas of research, such as chloroplast-mitochondrial interactions, phototransduction, and the behavioral response to light.