PARENT SESSION
Plenary Lectures 2
Tuesday August 31st, 2004 8:30 AM Room 210A

Crosstalk between the nuclear and chloroplast genetic systems. Jean-David Rochaix^*,1, David Dauvillée¹, Livia Merendno¹, Angela Falciatore¹, Frédy Barneche¹, Michel Goldschmidt-Clermont¹, ¹ Departments of Molecular Biology and Plant Biology, University of Geneva, Geneva, Switzerland

ABSTRACT- The biogenesis of the photosynthetic apparatus depends on the concerted action of the chloroplast and nucleo-cytosolic genetic systems. A large number of nucleus-encoded factors are specifically involved in several chloroplast post-transcriptional steps that include RNA processing, splicing and translation. We have recently identified and characterized several of these factors involved in the biosynthesis of photosystem I in Chlamydomonas. They are often part of multimeric RNA-protein complexes. As an example, the Tab1 and Tab2 proteins are specifically required for the translation of the psaB mRNA. Studies with chloroplast chimeric genes reveal that one target site of these factors is comprised within the psaB 5'UTR. The Tab2 amino-acid sequence displays significant sequence identity with several orthologs found only in eukaryotic and prokaryotic organisms performing oxygenic photosynthesis. Gel mobility shift assays reveal a direct and specific interaction between Tab2 and the psaB 5' UTR. We propose that Tab2 plays a key role in the initial steps of psaB mRNA translation and photosystem I assembly. Genetic interactions do not only operate from the nucleus to the chloroplast, but can also proceed in the opposite direction. The state of the chloroplast can profoundly affect nuclear gene expression. In particular, several observations indicate that chlorophyll precursors can act as plastid signals to regulate nuclear gene expression. We have identified two cDNAs with strong sequence similarity to the Arabidopsis FLU protein. They are generated through alternative splicing and encode two proteins, s-FLP and l-FLP (Flu-like proteins). The two mRNAs are weakly expressed in dark-adapted cells and are strongly induced by light. Moreover the FLPs are overexpressed in the dark in mutants deficient in chlorophyll synthesis. The ratio between the two FLP forms correlates with the appearance of specific porphyrin intermediates in the mutants. Both FLP proteins are imported into chloroplasts, associated with membranes and part of a large complex. However GST pull-down assays indicate that glutamyl-tRNA reductase strongly interacts with l-FLP and only poorly with s-FLP suggesting that these two FLPs are involved in different functions in Chlamydomonas.

KEY WORDS: gene expression, chloroplast, chlorophyll synthesis, Chlamydomonas