The thermodynamics and kinetics of electron transfer between cytochrome b6f and photosystem I in the chlorophyll d-dominated cyanobacterium, Acaryochloris marina.
We have investigated the photosynthetic properties of Acaryochloris marina, a cyanobacterium distinguished by having a high level of chlorophyll d, which has its absorption bands shifted to the red when compared with chlorophyll a. Despite this unusual pigment content, the overall rate and thermodynamics of the photosynthetic electron flow are similar to those of chlorophyll a-containing species. The midpoint potential of both cytochrome f and the primary electron donor of photosystem I (P(740)) were found to be unchanged with respect to those prevailing in organisms having chlorophyll a, being 345 and 425 mV, respectively. Thus, contrary to previous reports (Hu, Q., Miyashita, H., Iwasaki, I. I., Kurano, N., Miyachi, S., Iwaki, M., and Itoh, S. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 13319-13323), the midpoint potential of the electron donor P(740) has not been tuned to compensate for the decrease in excitonic energy in A. marina and to maintain the reducing power of photosystem I. We argue that this is a weaker constraint on the engineering of the oxygenic photosynthetic electron transfer chain than preserving the driving force for plastoquinol oxidation by P(740), via the cytochrome b(6)f complex. We further show that there is no restriction in the diffusion of the soluble electron carrier between cytochrome b(6)f and photosystem I in A. marina, at variance with plants. This difference probably reflects the simplified ultrastructure of the thylakoids of this organism, where no segregation into grana and stroma lamellae is observed. Nevertheless, chlorophyll fluorescence measurements suggest that there is energy transfer between adjacent photosystem II complexes but not from photosystem II to photosystem I, indicating spatial separation between the two photosystems.
Bailleul
Benjamin
B
Institut de Biologie Physico-Chimique, UMR 7141 CNRS-Université Paris 6, 13 Rue Pierre et Marie Curie, Paris 75005, France and the.
Johnson
Xenie
X
Institut de Biologie Physico-Chimique, UMR 7141 CNRS-Université Paris 6, 13 Rue Pierre et Marie Curie, Paris 75005, France and the.
Finazzi
Giovanni
G
Institut de Biologie Physico-Chimique, UMR 7141 CNRS-Université Paris 6, 13 Rue Pierre et Marie Curie, Paris 75005, France and the.
Barber
James
J
Division of Molecular Biosciences, Imperial College London, Biochemistry Building, South Kensington Campus, London SW7 2AZ, United Kingdom.
Rappaport
Fabrice
F
Institut de Biologie Physico-Chimique, UMR 7141 CNRS-Université Paris 6, 13 Rue Pierre et Marie Curie, Paris 75005, France and the. Electronic address: Fabrice.Rappaport@ibpc.fr.
Telfer
Alison
A
Division of Molecular Biosciences, Imperial College London, Biochemistry Building, South Kensington Campus, London SW7 2AZ, United Kingdom. Electronic address: a.telfer@imperial.ac.uk.
D001671:Biochemistry / Q000379:methods; D002734:Chlorophyll / Q000737:chemistry*; D000458:Cyanobacteria / Q000378:metabolism*; D045346:Cytochrome b6f Complex / Q000737:chemistry*; D004583:Electrons; D007700:Kinetics; D008027:Light; D008954:Models, Biological; D010084:Oxidation-Reduction; D045331:Photosystem I Protein Complex; D013050:Spectrometry, Fluorescence / Q000379:methods; D013816:Thermodynamics; D020524:Thylakoids / Q000378:metabolism; D013997:Time Factors
D045331:Photosystem I Protein Complex; D002734:Chlorophyll; C107509:chlorophyll d; D045346:Cytochrome b6f Complex