chlorophyll-a and 2-5-dimethyl-4-benzoquinone

Ala-251 in the membrane-parallel helix in the D-E loop of the D1 polypeptide close to the Q(B) pocket of photosystem II (PS II), was mutated to aspartate (D), lysine (K), leucine (L) or serine (S) in Synechocystis 6803. O2 evolution rates (H2O-->DCBQ; 2,6-dichloro-p-benzoquinone) of A251D, A251L and A251S were lower, being 38, 16, 62 and 70%, respectively, of that of the control, and there was an even more drastic impairment of O2 evolution when measured from H2O to DMBQ (2,5-dimethyl-p-benzoquinone), demonstrating modifications in the Q(B) pocket. However, in all other mutants but A251K, the Q(B) function could sustain O2 evolution at a level high enough to support photosynthetic growth. The mutant A251S, carrying a substitution of alanine for a chemically quite similar residue serine, was less severely affected. Substitution by a positively charged residue drastically delayed chlorophyll a fluorescence relaxation in the non-photosynthetic strain A251K, implying strong impairment of Q(A)-to-Q(B) electron transfer. Delay of fluorescence relaxation was clear in A251D as well, carrying a substitution of alanine for a negatively charged residue. The effects of the substitutions of A251 demonstrate the importance of this residue of the D1 polypeptide in the conformation of the acceptor side of PS II and, accordingly, the effect on the acceptor-side function of PS II was very clear. Nevertheless, the tolerance of PS II activity to high-light-induced photoinhibition in vivo and the subsequent D1 degradation were not much impaired in any of the photosynthetic mutant strains as compared to the control.

Topics: Alanine; Amino Acid Sequence; Amino Acid Substitution; Benzoquinones; Chlorophyll; Cyanobacteria; Cyclohexenes; Diuron; Glucose; Kinetics; Light; Light-Harvesting Protein Complexes; Models, Molecular; Molecular Sequence Data; Mutagenesis, Insertional; Mutagenesis, Site-Directed; Oxygen; Photosynthetic Reaction Center Complex Proteins; Photosystem II Protein Complex; Point Mutation; Protein Structure, Secondary; Recombinant Proteins

Upon illumination, a dark-adapted photosynthetic sample shows time-dependent changes in chlorophyll (Chl) a fluorescence yield, known as the Kautsky phenomenon or the OIDPS transient. Based on the differential effects of electron acceptors such as 2,5-dimethyl-p-benzoquinone (DMQ) and 2,6-dichloro-p-benzoquinone (DCBQ) on Chl a fluorescence transients of spinach thylakoids, we suggest that the OID phase reflects the reduction of the electron acceptor QA to QA- in the inactive PS II (see Graan, T. and Ort, D. (1986) Diochim. Biophys. Acta 852, 320-330). In spinach thylakoids, heat-induced increase of the Chl a fluorescence yield is also differentially sensitive to the addition of DMQ and DCBQ suggesting that this increase is mainly on the 'I' level, and thus heating is suggested to convert active PS II to inactive PS II centers. The kinetics of decay of QA-, calculated from variable Chl a fluorescence, was analyzed into three exponential components (365-395 microseconds; 6-7 ms; and 1.4-1.7 s). In heated samples, the decay rate of variable Chl a fluorescence is slower than the normal back-reaction rate; there is a preponderance of the slow component that may be due, partly, to the active centers undergoing slow back reaction between QA- and the S2 state of the oxygen-evolving complex.

Topics: Benzoquinones; Chlorophyll; Chloroplasts; Cyclohexenes; Hot Temperature; In Vitro Techniques; Intracellular Membranes; Light-Harvesting Protein Complexes; Photosynthetic Reaction Center Complex Proteins; Photosystem II Protein Complex; Plant Proteins; Plants; Quinones; Spectrometry, Fluorescence