Role of NAD(P)H:Quinone Oxidoreductase Encoded by drgA Gene in Reduction of Exogenous Quinones in Cyanobacterium Synechocystis sp. PCC 6803 Cells

I. V. Elanskaya^1*, V. G. Grivennikova², V. V. Groshev¹, G. V. Kuznetsova¹, M. E. Semina¹, and K. N. Timofeev³

¹Department of Genetics, ²Department of Biochemistry, and ³Department of Biophysics, Faculty of Biology, Lomonosov Moscow State University, Moscow 119992, Russia; fax: (7-095) 939-2957; E-mail: ivelanskaya@mail.ru

^* To whom correspondence should be addressed.

Received April 21, 2003; Revision received May 19, 2003

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Insertion mutant Ins2 of the cyanobacterium Synechocystis sp. PCC 6803, lacking NAD(P)H:quinone oxidoreductase (NQR) encoded by drgA gene, was characterized by higher sensitivity to quinone-type inhibitors (menadione and plumbagin) than wild type (WT) cells. In photoautotrophically grown cyanobacterial cells more than 60% of NADPH:quinone-reductase activity, as well as all NADPH:dinoseb-reductase activity, was associated with the function of NQR. NQR activity was observed only in soluble fraction of cyanobacterial cells, but not in membrane fraction. The effects of menadione and menadiol on the reduction of Photosystem I reaction center (P700⁺) after its photooxidation in the presence of DCMU were studied using the EPR spectroscopy. The addition of menadione increased the rate of P700⁺ reduction in WT cells, whereas in Ins2 mutant the reduction of P700⁺ was strongly inhibited. In the presence of menadiol the reduction of P700⁺ was accelerated both in WT and Ins2 mutant cells. These data suggest that NQR protects the cyanobacterial cells from the toxic effect of exogenous quinones by their reduction to hydroquinones. These data may also indicate the probable functional homology of Synechocystis sp. PCC 6803 NQR with mammalian and plant NAD(P)H:quinone oxidoreductases (DT-diaphorases).

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KEY WORDS: cyanobacterium Synechocystis sp. PCC 6803, NAD(P)H:quinone oxidoreductase, drgA gene, resistance to menadione, EPR-spectroscopy, P700⁺ reduction

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