Physiological Roles Of Ferredoxin-like Proteins In Synechocystis Sp.PCC 6803

Blue-green alga,also called cyanobacteria,are a diverse group of widespread prokaryotic microbes performing oxygenic photosynthesis.Some of cyanobacteria are capable of not only autotrophy via photosynthesis,but also facultative heterotrophy using external organic carbon sources.Ferredoxin?Fd?protein family consists of a variety of iron-sulfur proteins that participate photosynthetic linear electron transport,cyclic electron transport,and water-water cycle,thus play important roles in stress response in photosynthetic organisms.In addition,there are a sort of proteins homologous to Fd named as Fd-like proteins.These proteins are involved in many important biological functions in higher plants,such as chloroplast development.The significance of Fd-like proteins in cyanobacteria is not yet known.This thesis makes a primary study on physiological roles of two distinct Fd-like proteins in Synechocystis sp.PCC 6803.To investigate the role of Fd-like proteins,two knockout mutants??slr1205 and?ssr1041?were generated in Synechocystis.The growth under autotrophic,mixotrophic and photoherotrophic conditions of WT and two mutants were observed using a variety of different environmental conditions,including low temperature?18?and 23??and normal temperature?30??,high CO₂?3%?and low CO₂?air level?,light-dark cycle?2 h:2 h?and continuous light.It showed that no apparent difference was observed between the?ssr1041 and WT in all tested conditions,whereas various phenotypes were found in?slr1205.The growth rate of?slr1205 was faster than that of the WT under low temperature,but obviously slower under high CO₂ as well as mixotrophy and photoheterotrophy.However,there was no significant difference of?slr1205 and WT grown autotrophically under low CO₂.The respiration activity measured by oxygen electrode showed that there was no big difference between autotrophic cells of?slr1205 and WT,while remarkable decline was detected in mixotrophic and photoheterotrophic cells of?slr1205.The net photosynthesis activity was slower in?slr1205 cells grown either autotrophically,or mixotrophically.Additionally,the cells of?slr1205 showed higher carotenoid content and more yellowish color than WT.Also,the BN-PAGE demonstrated that the amount of NDH-1L complex,involved in cellular respiration,was significantly lower in?slr1205 than that in the WT grown photoheterotrophically.These data indicated a deficiency of inorganic and organic carbon source utilization in?slr1205,especially under conditions of adequate carbon sources.Quantitative proteomic analysis of?slr1205 and WT cells grown under mixotrophic condition revealed a significant downregulation of genes involved in nitrogen?e.g.sll1452 and sll1453?and sulfur?e.g.slr0902?metabolisms.Proteomic data also showed upregulation ofa-tocopherol biosynthesis gene slr1737 in?slr1205,which was in agreement with the higher carotenoid content.All our experimental data suggested that deletion of slr1205 encoding Fd-like protein led to a decrease of respiratory complex NDH-1L and slower respiration and net photosynthesis rates.Enhanced a-tocopherol biosynthesis cannot completely compensate the drawback of efficient carbon utilization.Lacking of Slr1205 also affected the nitrogen and sulfur metabolisms in cyanobacteria.Therefore,we postulated that Fd-like protein Slr1205 participates in the process of carbon,nitrogen,and sulfur metabolisms,and functions in optimization of photosynthesis and respiration in cyanobacteria.