Menaquinone as pool quinone in a purple bacterium

Abstract

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.

Fig. 1.

Quinone content of membranes and RCs from H. halophila. (A) UV-fluorescence quenching spots detected on TLC plates loaded with methanol/chloroform extracts of membranes and RC–LH complexes. The indicated bands contained redox active compounds identified as MK-8, MK-7, and UQ-8 by mass spectroscopy and quantified by UV-redox difference spectroscopy. (B) Flash-induced absorbance changes (measured at 423 nm) of the special pair pigment in membranes in the presence (filled circles) and absence (filled squares) of the Q_B-site inhibitor terbutryn as well as in RC–LH complexes (open triangles) or in RC complexes purified as published in ref. (closed triangles).

Fig. 2.

Schematic comparison of the redox properties of selected cofactors in the RC and the cyt bc₁ complex from H. halophila (this work) to the corresponding values in photosynthetic chains from other species based either on UQ- [exemplified by Rubrivivax gelatinosus (11, 36)] or MK- [represented by Heliobacillus chlorum (11, 23)] pools. The rectangles indicate the redox midpoint potentials of the respective cofactors on a redox scale.

Fig. 3.

Genomic organization and sequence characteristics of the cyt bc₁ complex in H. halophila. (A) Genomic arrangement of the genes coding for the 3 principal subunits of the cyt. bc₁ complex. “Y” denotes a sequence stretch that probably represents a fragment of a gene of the Rieske protein. (B) Sequence comparison of Y to the C-terminal end of the Rieske gene located distal from the cytb/cytc₁-diad. (C) Comparison of the Rieske protein's sequence range containing the second cluster-binding motif (shaded) and the 2 amino acid residues (shaded, arrows) considered to strongly influence the redox potential of the cluster. H. halophila is highlighted by a large shaded arrow, “α”–“ε” denote subgroups of the proteobacterial phylum and “E” stands for eukaryotic mitochondria. The sequences (accession numbers and parent species) used in the alignment are given in Table S1.

Fig. 4.

Distribution of MK- and UQ-based chains on a proteobacterial phylogenetic tree. This tree (reconstructed by the neighbor-joining method) is based on 16S r-RNA genes. The shaded area indicates extant and extinct lineages where both MK and UQ functioned alternatively. The probable evolutionary appearance of the UQ molecule after the divergence of the δ- and ε-subgroups and before the α/β/γ-radiation is indicated. The α, β, δ, and ε species used for the tree reconstruction are, Rhodobacter sphaeroides, Rubrivivax gelatinosus, EbS7, and Helicobacter pylori, respectively. Identification of HP and/or LP chains are based on our work for H. halophila, on refs. , , and for E. haloalkaliphila, on refs. , , and for A. vinosum, on ref. for Shewanella ANA-3, and on ref. for E. coli.