集胞藻 PCC 6803中Sll0875蛋白参与调控光系统I活性的生物功能研究

叶绿醌是由1个萘醌环和1个半不饱和植基侧链组成的一类光系统Ⅰ（photosystem Ⅰ，PSⅠ）特有的辅因子。目前，在蓝藻中对其生物合成途径的研究主要集中在萘醌环的形成方面，而对其植基侧链的合成尚缺乏相关报道。本研究通过与近期在拟南芥中发现的1种催化植基单磷酸形成植基二磷酸的激酶（VTE6）进行同源序列比对，在集胞藻 PCC 6803中发现1个与之高度同源的蛋白质Sll0875。研究发现，在Sll0875缺失突变体中，叶绿醌和生育酚的含量缺失，叶绿素的含量降低（P<0.05），且该突变体在无葡萄糖培养基中生长迟缓。进一步利用叶绿素荧光、P700氧化还原动力学、77K低温荧光光谱和免疫印迹分析等方法分析了该蛋白质的缺失对PSⅠ功能的影响。研究表明，在突变体Δsll0875中， PSⅠ活性下降，PSⅠ亚基含量与野生型相比显著降低（P<0.01）。这一结果表明，叶绿醌的缺失影响了PSⅠ复合物的累积，导致PSⅠ功能受损，从而影响了蓝藻正常的生长和发育。本研究在蓝藻中证实植醇磷酸化途径对叶绿醌合成的重要性，为进一步研究蓝藻中叶绿醌在PSⅠ复合物的合成、组装和稳定等过程中的作用奠定基础。

Functional Analysis of Sll0875 in the Regulation of PSI Activity in Synechocystis sp. PCC 6803

Phylloquinone is a unique cofactor of photosystem Ⅰ (PSⅠ), made up of a redox active naphthoquinone ring attached to a partially saturated C-20 phytyl side chain. At present, the research on the biosynthesis of phylloquinone in cyanobacteria is mainly focused on the formation of naphthoquinone ring, while there was a shortage of reports in the biosynthesis of phytyl side chain. In this study, a highly homologous protein Sll0875 was found in Synechocystis sp. PCC 6803 by homologous sequence alignment with VTE6, a kinase involved in phylloquinone biosynthesis by converting phytyl-phosphate into phytyl-diphosphate in Arabidopsis thaliana. The resulting Sll0875 mutant, called Δsll0875,accumulates none phylloquinone and tocopherol, as well as low amounts of chlorophyll content (P<0.05). The mutant had retarded growth in the absence of added glucose. Chlorophyll fluorescence, P700 absorbance changes, 77 K fluorescence emission spectra and Western blot analyses showed that in Δsll0875, PSⅠ function was impaired and accumulation of the PSⅠ complex was reduced remarkably (P<0.01), indicating that phylloquinone deficiency affected PSⅠ function, thus hindering the normal growth of cyanobacteria. Our results provide the evidence that the phytol phosphorylation pathway is essential for phylloquinone biosynthesis in cyanobacteria for the first time, and a basis for further investigate the protein synthesis, assembly and stability of PSⅠ complex in cyanobacteria.