Professor The University of Tennessee Knoxville, Tennessee
Body of Abstract: The eS6 protein, ribosomal protein of the small subunit 6, is the target of dynamic phosphorylation events, which integrate numerous metabolic and environmental signals, from light and the circadian clock to metabolism and abiotic stress. The functional consequences of eS6-phosphorylation are, however, incompletely understood, a situation mirrored in yeast and animals. We generated the first set of plants in which eS6-phosphorylation is fully disrupted and studied the phenotypic consequences at the level of translation, ribosome biogenesis, gene expression, photosynthesis, and seedling development. Compared to plants rescued with a phospho-enabled version of eS6, the eS6-phospho-deficient Arabidopsis plants were fully viable and fertile and developed normally throughout most of their life cycle. The phospho-deficient eS6 rescued most defects seen in rps6 mutant plants. Rescued were immediate biochemical functions of eS6 in ribosome biogenesis and polyribosome loading, specifically the dramatic translational boost after onset of light in the morning, as well as downstream functions in photosynthesis and plant growth. However, the eS6 phospho-deficient plants displayed subtle defects. Their transcriptome as well as their proteome were altered, impacting cytokinesis and ribosome biogenesis, which may point to a potential molecular function of this highly dynamic phosphorylation event. Further exemplified by residual growth phenotypes. i.e. ‘pointed first leaves’ and ‘asymmetric cotyledons’, the few defects that were observed were mild versions of phenotypes also seen in rps6 null mutants, indicating that they are loss-of-function rather than gain-of function events. These findings indicate that the highly regulated phosphorylation in the C-terminal tail of eS6 is not essential for most functions of the eS6 protein in protein synthesis, plant development and growth in the laboratory. Supported byNIH 2R15GM129672.