PhotosynthesisResearch 45: 31-40, 1995. © 1995 KluwerAcademic Publishers. Printed in theNetherlands. Regular paper Interaction of the photosynthetic and respiratory electron transport chains producing slow 02 signals under flashing light in Synechocystis sp. PCC 6803 Pascal C. Meunier 1, Robert L. Burnap 2 & L. A. Sherman 1 1 Department of Biological Sciences, Purdue University, West Lafayette, IN 47907, USA (address for correspondence and reprints); 2Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, OK 74078, USA Received28 October1994;accepted in revisedform 1 June 1995 Key words: Synechocystis sp. PCC 6803, cyanobacteria, respiration, ADRY effect Abstract We investigated the slow signal of apparent 02 release under brief light flashes by using mutants of Synechocystis sp. PCC 6803 which lacked CP43 and D1. The slow signal was present at higher amplitudes in the mutants. It was inhibited by starving the mutants of glucose (>90%), by 10 mM NaN 3 (85%) and by boiling samples for 2 min (100%). In the mutants and in the wild-type, the slow signal was 95% inhibited by the combination of DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone) and HQNO (2-n-heptyl-4-hydroxyquinoline-N- oxide). In the wild type, the addition of DCMU (3-(3,4-dichlorophenyl)-l,l-dimethylurea) or CCCP (carbonyl- cyanide m-chlorophenylhydrazone) completely inhibited photosynthetic 02 evolution, yet failed to inhibit the slow signal. We explain the kinetics of the wild-type signal as a positive deflection due to the inhibition of respiration by PS I activity, and a negative deflection due to the stimulation of respiration by electrons originating from PS II. We found no evidence of a 'meta-stable $3' in Synechocystis sp. PCC 6803 that could contribute to the slow signal of apparent 02 release. We present a calculation which involves only averaging, division and subtraction, that can remove the contribution of the slow signal from the true photosynthetic 02 signal and provide up to a 10-fold improved accuracy of the S-state models. Abbreviations: ADRY - Acceleration of the Deactivation Reactions of the water-splitting enzyme sys- tem Y; Ant-2-p - 2-(3-chloro-4-trifluoromethyl)-anilino-3,5-dinitrothiophene; CCCP - carbonylcyanide m- chlorophenylhydrazone; DBMIB- 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, a.k.a. Dibromothymo- quinone; DCMU - 3-(3,4-dichlorophenyl)- 1,1-dimethylurea (diuron); HQNO - 2-n-heptyl-4-hydroxyquinoline-N- oxide; S. 6803 - Synechocystis sp. PCC 6803 Introduction Cyanobacteria are prokaryotes capable of oxygenic photosynthesis. The structure ofphotosystem II (PS II) is similar in cyanobacteria and higher plants, except for different extrinsic proteins, such as phycobili- somes. In addition, cyanobacterial PS II centers have a cytochrome c-550 and a 9-12 kDa extrinsic polypep- tide (Stewart and Bendall 1989; Shen et al. 1992; Shen and Inoue 1993), instead of the 24 and 17 kDa polypeptides, as well as the manganese-stabilizing pro- tein (MSP) which is also present in higher plants. The extrinsic PS II proteins provide an optimized environ- ment for water splitting by the manganese complex of PS II. The redox states of this manganese complex are known as S-states; the most oxidized S-state is $4, which spontaneously returns to So when 02 is released. The So state is oxidized to Sl, $2, $3 and again up to $4. In higher plants, the $2 and $3 states are unstable in the dark, and deactivate to $1 with a lifetime dependent on the redox state of the PS II electron acceptors (Kok et al. 1970; Styring and Rutherford 1987). Thus, no 02 is