| Selenium (Se), an essential micronutrient for animals, humans and microorganisms, is also a beneficial nutrient for growth and development of many plants. Selenoproteins and selenium polysaccharide are involved in cell metabolism. Although Se is widely distributed in nature, variation of Se concentration in different areas and of different capacities on Se metabolism for different organisms had been noticed.Rhodobacter sphaeroides, one of the widely distributed photosynthetic bacterium, can tolerate high concentration of selenate and accumulate Se0inside cells. Analysis of its genome indicated that metabolism of selenate is mainly catalyzed by sulfate activating complexe (SAC) which consists the activities of ATP sulfurylase (ATPS) and Adenosine5’-phosphosulfate kinase (APSK) and had been shown to channel APS between these two active sites.In order to study the roles of SAC in selenate metabolism, activation and reduction pathway of selenate by SAC and GSH were analyzed, and E. coli mutant whose sac was replaced by that from RS was constructed and analyzed. Following results were concluded.1. Catalytic efficiency (V/Km) of SAC on selenate was higher than that on sulfate (7.7times).2. APSe channeling was not detected, although SAC channels APS.3. Under different conditions, activated selenate APSe could be hydrolyzed to form AMP and selenate, be further activated by APSK to form PAPSe, be reduced by Adenosine5’-phosphosulfate reductase (APSR) to selenite, and be reduced to orange-red color of Se0by GSH.4. In the absence of APSR, SAC APSK could phosphorylate all APSe formed into PAPSe.5. PAPSe could be hydrolyzed into PAP and selenate, or be reduced into orange-red color of Se0by GSH.6. Replacement of cysD in E.coli by Rhodobacter sphaeroides sac did not increase its selenate tolerant ability, and decreased sulfate assimilation ability.These work laid a solid foundation for further analysis on mechanism of selenate metabolism by Rhodobacter sphaeroides. |
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