The menD and menE homologs code for 2-succinyl-6-hydroxyl-2,4-cyclohexadiene-1-carboxylate synthase and O-succinylbenzoic acid-CoA synthase in the phylloquinone biosynthetic pathway of Synechocystis sp. PCC 6803

The genome of the cyanobacterium Synechocystis sp. PCC 6803 contains genes identified as menD and menE, homologs of Escherichia coli genes that code for 2-succinyl-6-hydroxyl-2,4-cyclohexadiene-1-carboxylate (SHCHC) synthase and O-succinylbenzoic acid-CoA ligase in the menaquinone biosynthetic pathway. In cyanobacteria, the product of this pathway is 2-methyl-3-phytyl-1,4-naphthoquinone (phylloquinone), a molecule used exclusively as an electron transfer cofactor in Photosystem (PS) I. The menD^- and menE^- strains were generated, and both were found to lack phylloquinone. Hence, no alternative pathways exist in cyanobacteria to produce O-succinylbenzoyl-CoA. Q-band EPR studies of photoaccumulated quinone anion radical and optical kinetic studies of the P700⁺ [F_A/F_B]^- backreaction indicate that in the mutant strains, plastoquinone-9 functions as the electron transfer cofactor in the A₁ site of PS I. At a light intensity of 40 μE m^-2 s^-1, the menD^- and menE^- mutant strains grew photoautotrophically and photoheterotrophically, but with doubling times slower than the wild type. Both of which are sensitive to high light intensities. Low-temperature fluorescence studies show that in the menD^- and menE^- mutants, the ratio of PS I to PS II is reduced relative to the wild type. Whole-chain electron transfer rates in the menD^- and menE^- mutant cells are correspondingly higher on a chlorophyll basis. The slower growth rate and high-light sensitivity of the menD^- and menE^- mutants are therefore attributed to a lower content of PS I per cell.