Cyanobacterial quinomics studies of quinones in cyanobacteria /

by 1973- Sakuragi, Yumiko

iii Roles and functions of isoprenoid quinones (phylloquinone, plastoquinone-9) and ?-tocopherol were investigated in cyanobacteria. Comparative genome analyses of 14 cyanobacteria suggested that phylloquinone (PhyQ) biosynthesis in most but not all cyanobacteria occurs similarly to menaquinone biosynthesis in Escherichia coli. This was further supported by the discovery that two cyanobacteria, Synechococcus sp. PCC 7002 and Gloeobacter violaceus PCC 7421, synthesize menaquinone-4 (MQ-4). Targeted inactivation of the menB, menF, and menG genes resulted in the incorporation of plastoquinone-9 (PQ-9) and demethyl-MQ or demethyl-PhyQ into Photosystem I (PS I) complexes. In the PS I complexes containing demethyl-PhyQ, the rate of electron transfer from A1 to the iron-sulfur clusters slowed by a factor of two, while the kinetics of the P700+ [FA/FB]- backreaction increased by a factor of 3 to 4. These results were explained by a lowering of the equilibrium constant between Q -/Q and FX-/FX in the demethyl-PhyQ containing PS I complexes by a factor of ~10. Populations of ?-tocopherol mutants of the cyanobacterium Synechocystis sp. PCC 6803, previously isolated in the presence of glucose, were found to be phenotypically and genotypically heterogeneous. Newly isolated, “authentic” tocopherol mutants were unable to grow in the presence of glucose at pH 7.0; this was suggested to be due to a significant reduction of the amounts of sigA and rbcL transcripts in cells under these conditions. The slr2031 product, which has been previously shown to be involved in sulfur, nitrogen, and carbon metabolism, and genes encoding inorganic carbon uptake mechanisms, were found to be constitutively down-regulated in the iv “authentic” tocopherol mutants. The results indicate that ?-tocopherol is involved in the transcriptional regulation of these metabolic genes and plays an important role in the coordination of nitrogen, sulfur, and carbon metabolism in Synechocystis sp. PCC 6803. The PQ-9 biosynthesis pathway was predicted to be similar to that for ubiquinone biosynthesis based on comparative genome analyses of 14 cyanobacteria. However, targeted inactivation mutagenesis of eight genes encoding putative methyltransferase genes similar to UbiE/MenG in E. coli did not affect PQ-9 biosynthesis in Synechocystis sp. PCC 6803. Based on the results obtained, a possible PQ-9 biosynthesis pathway is proposed.