Genes encoding A-type flavoproteins are essential for photoreduction of O2 in cyanobacteria

Yael Helman, Dan Tchernov, Leonora Reinhold, Mari Shibata, Teruo Ogawa, Rakefet Schwarz, Itzhak Ohad, Aaron Kaplan*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

220 Scopus citations


O2 photoreduction by photosynthetic electron transfer, the Mehler reaction [1], was observed in all groups of oxygenic photosynthetic organisms [2-4], but the electron transport chain mediating this reaction remains unidentified. We provide the first evidence for the involvement of A-type flavoproteins that reduce O2 directly to water in vitro. Synechocystis sp. strain PCC 6803 mutants defective in flv1 and flv3, encoding A-type flavoproteins, failed to exhibit O2 photoreduction but performed normal photosynthesis and respiration. We show that the light-enhanced O2 uptake was not due to respiration or photorespiration. After dark acclimation, photooxidation of P700 was severely depressed in mutants Δflv1 and Δflv3 but recovered after light activation of CO2 fixation, which gives P700 an additional electron acceptor. Inhibition of CO2 fixation prevented recovery but scarcely affected P700 oxidation in the wild-type, where the Mehler reaction provides an alternative route for electrons. We conclude that the source of electrons for O2 photoreduction is PSI and that the highly conserved A-type flavoproteins Flv1 and Flv3 are essential for this process in vivo. We propose that in cyanobacteria, contrary to eukaryotes, the Mehler reaction produces no reactive oxygen species and may be evolutionarily related to the response of anaerobic bacteria to O2.

Original languageAmerican English
Pages (from-to)230-235
Number of pages6
JournalCurrent Biology
Issue number3
StatePublished - Feb 2003

Bibliographical note

Funding Information:
We thank Professor W. Vermaas for kindly providing us with mutant ΔctaDIEI/ctaDIIEII/cydAB of Synechocystis PCC 6803. This research was supported by grants from the Israel Science Foundation (ISF) to I.O. and to A.K.; program MARS2 (the German BMBF and the Israeli Ministry of Science) to A.K. and L.R.; and a Grant-in-aid for Scientific Research (B) (2)(12440228) to T.O.


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