PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

Post-transcriptional control of LHCII protein expression in Chlamydomonas reinhardtii. Jan Mussgnug¹, Mary Hamilton², Conrad Mullineaux³, Michael Hippler⁴, Jörg Nickelsen⁵, Peter Nixon², Olaf Kruse^*,1, ¹ University of Bielefeld, Department of Biology, Bielefeld, Germany² Imperial College London, Department of Biological Sciences, London, U.K.³ University College London, Department of Biology, London, U.K.⁴ University of Pennsylvania, Department of Biology, Philadelphia, USA⁵ University of Bochum, Department of Biology, Bochum, Germany

ABSTRACT- Green plants respond to changes in ambient light by modulating the relative sizes of their light harvesting antenna systems (LHC). Two major adaptation mechanisms, LHC state transitions and LHC expression control, enables the organism to adapt its antenna systems to sub-optimal light conditions. We have applied a chlorophyll fluorescence based screen to identify nuclear insertion mutants of the green alga Chlamydomonas reinhardtii that are impaired in state transitions. The disrupted gene of one of these mutants, stm3, was found to be encoding for a cytosolic RNA-binding protein (NAB1,AccNo.AY157846). The loss of NAB1 in stm3 caused a disruption of LHCBM protein expression control leading to higher expression rates of LHCBM6 and LHCBM2/8 proteins. The altered LHCII antenna structure in stm3 caused severe structural changes of the thylakoid granal membranes. Complementation experiments confirmed that the disruption of Nab1 in stm3 was entirely responsible for the observed phenotype. Western blots with anti-NAB1 and RNA-binding studies with over-expressed NAB1 and isolated Lhcbm6 mRNA identified NAB1 as a light-driven RNA-binding protein with a high binding affinity to a Lhcbm6-RNA consensus motif. This domain was previously identified as a RNA-binding protein recognition site in the NAB1 homologue FRGY2 and exists with different levels of identity on all seven Lhcbm mRNAs in C. reinhardtii. We conclude from our results that NAB1 is essential for a functional control of the post-transcriptional expression of light harvesting antenna LHCBM iso-forms under transient light changes. Overall our data identify for the first time a translational control mechanism for the light harvesting antenna as a cytosolic fine tuning system independent of the redox-regulated cab gene transcription control in the nucleus.

KEY WORDS: RNA binding, light acclimation, LHCII, translation control