Cyanobacteria use both p-hydroxybenozate and homogentisate as a precursor of plastoquinone head group

Until recently it was believed that cyanobacterial pathway of plastoquinone biosynthesis is analogical to that of higher plants. In plants, homogentisate is a precursor of the hydrophilic head group of plastoquinone. Recent experiments on Synechocystis sp. PCC 6803 have shown that this organism takes advantage of another pathway that resembles ubiquinone biosynthetic pathway of alpha-, beta- and gamma-proteobacteria. In the present work, we have analysed the content of plastoquinone, tocopherol and tocopherolquinone in six strains of cyanobacteria and compared the obtained results with search for genes of homologues of enzymes participating in tocopherol and ubiquinone biosynthesis. We have shown that inhibition of homogentisate synthesis lowers tocopherol content but does not affect plastoquinone synthesis in Synechococcus sp. PCC 7002. Inhibitors of p-hydroxybenzoate and homogentisate prenyltransferases selectively influenced plastoquinone and tocopherol biosynthesis in Synechocystis sp. PCC 6803. Radiolabelled C-14-p-hydroxybenzoate was incorporated into plastoquinone by three cyanobacteria species investigated. Although, when C-14-homogentisate was added to growth medium, the labelled plastoquinone was found in extracts of the cyanobacteria. Synechocystis sp. PCC 6803 grown in the presence of C-14-homogentisate showed also small amounts of the labelled tyrosine, suggesting that cyanobacteria are able to incorporate exogenously added homogentisate into shikimate pathway.