Abstract

The biogenesis of the photosynthetic apparatus of algae and land plants depends on the concerted interactions of the nuclear and chloroplast genetic systems. A large number of nucleus-encoded factors are involved in post-transcriptional steps of chloroplast gene expression including RNA processing and splicing, translation and assembly of the photosynthetic complexes. One example is provided by the translation factor Tab2/ATAB2 which is conserved in oxygenic photosynthetic organisms. Considering its role in protein synthesis and its photoreceptor-mediated expression, ATAB2 represents a novel factor in the signalling pathway of light-controlled translation of photosystem proteins during early plant development. Photosynthetic organisms are able to adjust to changes in light quality and to maintain a high photosynthetic yield and energy homeostasis through state transitions. This process involves a reorganization of the light-harvesting system (LHCII) in the thylakoid membranes and the balancing of light excitation energy between PSII and PSI . Excess stimulation of PSII relative to PSI leads to the reduction of the plastoquinone pool and to the activation of a kinase, to the phosphorylation of LHCII and to the displacement of LHCII from PSII to PSI (state 2). Excess stimulation of PSI leads to the reverse process (state 1). We have used a genetic approach in Chlamydomonas with the ultimate aim of identifying some of the key components involved in the signaling chain of state transitions. In this way the thylakoid-associated Ser-Thr kinase Stt7 was identified which is required for the phosphorylation of LHCII and for state transitions. It belongs to a small family of protein kinases, also conserved in land plants, which are part of a regulatory circuitry apparently involved both in short term and long term acclimation. http://www.molbio.unige.ch/rochaix/index.php