Sucrose dependent translational dynamics in Arabidopsis thaliana

Sucrose dependent translational dynamics Gene expression is regulated at several different levels starting from chromatin remodeling and transcription in the nucleus to translation and post-translational modifications in the cytosol. Depending on the gene and circumstances, different regulatory mechanisms are operational at various levels. mRNA translation is regulated both at the level of specific mRNAs and at the global level, where thousands of mRNA transcripts are affected. In yeast, sugar depletion results in inhibition of ribosome biogenesis and mRNA translation. In A. thaliana similar conditions results in down regulation of many r-protein genes and factors involved in ribosome biogenesis, indicating that general mRNA translation in A. thaliana likely is similarly controlled. In A. thaliana leaves and cell cultures, diurnal sugar levels and sugar starvation regulate gene expression at the transcriptional and at the translation level. In this thesis the effects of high sucrose levels on translational dynamics is investigated both at the gene specific level as well as at the global level. It is concluded that sucrose affects gene expression to a much larger extent than that was previously anticipated. Using an mRNA profiling approach, translational regulation in response to high sucrose levels was studied. Total and polysomal mRNA populations were isolated from A. thaliana leaves incubated in high sucrose levels or control conditions. It is shown that in response to high sucrose levels many genes are translationally controlled. Substantially more genes change their presence in the polysomal mRNA population in response to sucrose compared to sucrose regulated genes in the total mRNA population. Interestingly, many of the translationally controlled genes enriched in the polysomal mRNA population are known sugar regulated genes. These genes encode proteins involved in photosynthesis, ion and lipid metabolism and translation. These results suggest that sucrose-regulated genes are preferentially selected on or excluded from polysomes. mRNA selection is influenced by the ribosome and associated non-ribosomal proteins. Sucrose affects gene expression of many factors involved in mRNA translation, which includes genes that encode ribosomal proteins (r-proteins), but also other RNA binding proteins. This indicates that the general translation machinery is affected by high sucrose levels. Ribosomes are heterogeneous, large protein complexes consisting of 79 different r-proteins. In A. thaliana, each r-protein is encoded by two to six different genes. Using an advanced LC-MSE proteomics approach, the relative abundance of r-proteins in polysome protein extracts was quantified. About 71 % of the 234 expected r-proteins were identified and are present in ribosomal preparations. The abundance of specific r-protein paralogs changes significantly in response to high sucrose levels. This suggests that different r-protein paralogs are incorporated in the ribosome upon high sucrose levels and that, therefore, plants are capable of changing their ribosomal heterogeneity in response to environmental stimuli. Possibly, the differential incorporation of r-protein within the ribosome affects ribosome kinetics, accuracy or cellular localization. Only few reports have appeared on the function of r-protein paralogs, but future studies on the functions of individual r-protein paralogs is required to fully understand their biological roles.