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

Beatrycze Nowicka,Jerzy Kruk

doi:10.1007/s11738-015-2043-0

Abstract

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 α-, β- and γ-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 14C-p-hydroxybenzoate was incorporated into plastoquinone by three cyanobacteria species investigated. Although, when 14C-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 14C-homogentisate showed also small amounts of the labelled tyrosine, suggesting that cyanobacteria are able to incorporate exogenously added homogentisate into shikimate pathway.

Highlights

Isoprenoid quinones and chromanols are important groups of amphipathic compounds, both containing isoprenoid side-chain and sharing some common steps in their biosynthetic pathways
Two other species chosen were: Thermosynechococcus elongatus, in which there are no close homologues of genes encoding enzymes of plant a-Toc and PQ biosynthetic pathway and Synechococcus sp
Analysis of prenyllipids and bioinformatic analysis of genes encoding enzymes required for PQ and UQ biosynthesis in selected strains of cyanobacteria were performed

Summary

Introduction

Isoprenoid quinones and chromanols are important groups of amphipathic compounds, both containing isoprenoid side-chain and sharing some common steps in their biosynthetic pathways. Plastoquinone (PQ), present in all cyanobacteria, participates both in photosynthetic and respiratory electron transport chains (Nowicka and Kruk 2010). This compound shows antioxidant properties, especially in the reduced form (Nowicka and Kruk 2010). A-Tocopherol (a-Toc), a well-known antioxidant, occurs in majority of cyanobacteria, there are species lacking it, e.g., Synechococcus elongatus sp. Among oxidation products of a-Toc, a-tocopherolquinone (a-TQ) can be found in minor amounts in some species of cyanobacteria (Carr and Hallaway 1965)

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Conclusion