RESUMEN
Phylloquinone is a unique cofactor of photosystem I (PS I ) , 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 S110875 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 S110875 mutant, called As/Z0875, 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 As/Z0875, PS I function was impaired and accumulation of the PS I complex was reduced remarkably (P<0.01), indicating that phvlloquinone deficiency affected PS I 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 PS I complex in cyanobacteria.