Copyright © Physiologia Plantarum 1999 PHYSIOLOGIA PLANTARUM 105: 670 – 678. 1999 Printed in Ireland —all rights resered ISSN 0031-9317 Flavodoxin accumulation contributes to enhanced cyclic electron flow around photosystem I in salt-stressed cells of Synechocystis sp. strain PCC 6803 Martin Hagemann a, *, Robert Jeanjean b , Sabine Fulda a , Michel Havaux c , Francoise Joset b and Norbert Erdmann a a Uniersita ¨t Rostock, FB Biologie, Doberaner Str. 143, D-18051 Rostock, Germany b LCB-CNRS, 31 Chemin J. Aiguier, 13402 Marseille Cedex 20, France c CEA/Cadarache, DSV, DEVM, Laboratoire d’Ecophysiologie de la Photosynthe `se, 13108 Saint -Paul -lez -Durance, France *Corresponding author, e -mail: mh@boser.bio4.uni -rostock.de Received 28 August 1998; revised 30 November 1998 raised salt concentrations and, as expected, to low iron contents. The salt-regulated accumulation of flavodoxin encoded by the Salt-acclimated cells of wild type (WT) Synechocystis display isiB gene and its possible function were investigated in the increased activity of photosystem I (PSI), primarily used for cyanobacterium Synechocystis sp. strain PCC 6803. In North- increased cyclic electron transport capacity (Jeanjean et al. ern blot experiments, a slight increase of the isiB -specific mRNA 1993, Plant Cell Physiol 34: 1073 – 1079). In salt-acclimated was observed in salt-shocked and salt-acclimated cells. High levels of flavodoxin protein were detected in cells acclimated to cells of the flavodoxin null mutant, the level of cyclic electron 342 mM NaCl. In order to analyze the function of flavodoxin flow was lower than in wild type cells. It was concluded that flavodoxin plays a role as an alternative electron carrier, used in cyanobacterial salt acclimation, an insertion null mutant of isiB was constructed. It was possible to adapt this mutant to for cyclic electron flow in salt-treated Synechocystis cells. Introduction Cyanobacteria are photoautotrophic organisms performing oxygen-evolving photosynthesis like higher plants. As a result of their 3 billion years of evolution, cyanobacteria are presently found in almost all ecosystems, demonstrating their remarkable capability for environmental acclimation. Accli- mation to high or changing salinities includes two main physiological processes in cyanobacteria: (1) active ion ex- port; (2) accumulation of osmoprotective compounds. Syn- thesis of salt stress-specific proteins allow these processes to be performed (Joset et al. 1996). The first process, in partic- ular, expresses an enhanced demand of energy. Therefore, it is not surprising that salt-induced changes in the cyanobacte- rial photosynthetic activity and respiration have been studied. A slightly enhanced content and a doubling of activity of photosystem I (PSI) have been reported to be characteristic for salt-loaded cells of Synechocystis sp. PCC 6803 and other strains (Jeanjean et al. 1993, Schubert et al. 1993, Hibino et al. 1996, Murakami et al. 1997). These changes lead to an enhanced cyclic electron flow around PSI (Hibino et al. 1996, Jeanjean et al. 1998), which facilitates higher ATP produc- tion, necessary for primary and secondary active ion export mechanisms. Furthermore, an increase in respiratory oxygen consumption, which is accompanied by enhanced activity and content of cytochrome oxidase, was also often found in salt-stressed cyanobacterial cells (Gabbay-Azaria et al. 1992, Peschek et al. 1994). A protein which accumulated in salt-acclimated cells of Synechocystis, could be identified as a flavodoxin (Fulda and Hagemann 1995). Flavodoxin is a small soluble electron carrier, which can functionally replace ferredoxin for almost all of its functions (e.g. photosynthetic electron transport, nitrate reduction, fatty acid desaturation) (Straus 1994). Only glutamate synthase (GltS) seems to be solely dependent on ferredoxin in Synechocystis (Schmitz et al. 1996). In cells of Synechococcus sp. strain PCC 7002, a close binding of flavodoxin to PSI has been demonstrated by chemical Abbreiations – Chl a : chlorophyll a ; DCMU: 3-(3,4-dichlorophenyl)-1,1-dimethylurea; FNR: ferriedoxin/NADP + -oxidoreductase; Km: kanamycin; NDH: NAD(P)H-dehydrogenase; PSI: photosystem I; PSII: photosystem II; WT: wild type. Physiol. Plant. 105, 1999 670