Photosynthesis and Cell Growth Trigger Degradation of Phycobilisomes during Nitrogen Limitation

Under nitrogen (N)-limited conditions, the non-N2-fixing cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis 6803) actively grows during early stages of starvation by performing photosynthesis but gradually stops the growth and eventually enters dormancy to withstand long-term N limitation. During N limitation, Synechocystis 6803 cells degrade the large light-harvesting antenna complex phycobilisomes (PBSs) presumably to avoid excess light absorption and to reallocate available N to essential functions for growth and survival. These two requirements may be driving forces for PBS degradation during N limitation, but how photosynthesis and cell growth affect PBS degradation remains unclear. To address this question, we examined involvements of photosynthesis and cell growth in PBS degradation during N limitation. Treatment of photosynthesis inhibitors and shading suppressed PBS degradation and caused non-bleaching of cells during N limitation. Limitations of photosynthesis after initial gene responses to N limitation suppressed PBS degradation, implying that photosynthesis affects PBS degradation in a post-translational manner. In addition, limitations of cell growth by inhibition of peptidoglycan and fatty acid biosynthesis, low growth temperature and phosphorous starvation suppressed PBS degradation during N limitation. Because decreased photosynthetic activity led to decreased cell growth, and vice versa, photosynthesis and cell growth would inseparably intertwine each other and affect PBS degradation during N limitation in a complex manner. Our data shed light on the coordination mechanisms among photosynthesis, cell growth and PBS degradation during N limitation.