Menaquinone as pool quinone in a purple bacterium.
Purple bacteria have thus far been considered to operate light-driven cyclic electron transfer chains containing ubiquinone (UQ) as liposoluble electron and proton carrier. We show that in the purple gamma-proteobacterium Halorhodospira halophila, menaquinone-8 (MK-8) is the dominant quinone component and that it operates in the Q(B)-site of the photosynthetic reaction center (RC). The redox potentials of the photooxidized pigment in the RC and of the Rieske center of the bc(1) complex are significantly lower (E(m) = +270 mV and +110 mV, respectively) than those determined in other purple bacteria but resemble those determined for species containing MK as pool quinone. These results demonstrate that the photosynthetic cycle in H. halophila is based on MK and not on UQ. This finding together with the unusual organization of genes coding for the bc(1) complex in H. halophila suggests a specific scenario for the evolutionary transition of bioenergetic chains from the low-potential menaquinones to higher-potential UQ in the proteobacterial phylum, most probably induced by rising levels of dioxygen 2.5 billion years ago. This transition appears to necessarily proceed through bioenergetic ambivalence of the respective organisms, that is, to work both on MK- and on UQ-pools. The establishment of the corresponding low- and high-potential chains was accompanied by duplication and redox optimization of the bc(1) complex or at least of its crucial subunit oxidizing quinols from the pool, the Rieske protein. Evolutionary driving forces rationalizing the empirically observed redox tuning of the chain to the quinone pool are discussed.
Laboratoire de Bioénergétique et Ingénierie des Protéines, Unité Propre de Recherche 9036, Institut Fédératif de Recherche 88, Centre National de la Recherche Scientifique, F-13402 Marseille Cedex 20, France. schoepp@ifr88.cnrs-mrs.fr
Barbara
B
Schoepp-Cothenet
Clément
C
Lieutaud
Frauke
F
Baymann
André
A
Verméglio
Thorsten
T
Friedrich
David M
DM
Kramer
Wolfgang
W
Nitschke
D000595:Amino Acid Sequence; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D016680:Genome, Bacterial / Q000235:genetics; D008969:Molecular Sequence Data; D010758:Phosphorus / Q000378:metabolism; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism; D011485:Protein Binding; D020560:Proteobacteria / Q000235:genetics / Q000378:metabolism* / Q000528:radiation effects; D011809:Quinones / Q000378:metabolism*; D016415:Sequence Alignment; D014812:Vitamin K / Q000378:metabolism*
D045322:Photosynthetic Reaction Center Complex Proteins; D011809:Quinones; D014812:Vitamin K; D010758:Phosphorus; D003577:Cytochrome P-450 Enzyme System