Journal
BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS
Volume 1557, Issue 1-3, Pages 67-76Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/S0005-2728(02)00396-1
Keywords
phylloquinone; Photosystem I; Synechocystis; plastoquinone; menD; menE
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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 A(1) site of PS I. At a light intensity of 40 muE 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. (C) 2002 Elsevier Science B.V All rights reserved.
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