Multiple Rieske/cytb complexes in a single organism.

Most organisms contain a single Rieske/cytb complex. This enzyme can be integrated in any respiratory or photosynthetic electron transfer chain that is quinone-based and sufficiently energy rich to allow for the turnover of three enzymes - a quinol reductase, a Rieske/cytb complex and a terminal oxidase. Despite this universal usability of the enzyme a variety of phylogenetically distant organisms have multiple copies thereof and no reason for this redundancy is obvious. In this review we present an overview of the distribution of multiple copies among species and describe their properties from the scarce experimental results, analysis of their amino acid sequences and genomic context. We discuss the predicted redox properties of the Rieske cluster in relation to the nature of the pool quinone. It appears that acidophilic iron-oxidizing bacteria specialized one of their two copies for reverse electron transfer, archaeal Thermoprotei adapted their three copies to the interaction with different oxidases and several, phylogenetically unrelated species imported a second complex with a putative heme ci that may confer some yet to be determined properties to the complex. These hypothesis and all the more the so far completely unexplained cases call for further studies and we put forward a number of suggestions for future research that we hope to be stimulating for the field. This article is part of a Special Issue entitled: Respiratory complex III and related bc complexes.

Comment on "A bacterium that can grow by using arsenic instead of phosphorus".

Wolfe-Simon et al. (Research Articles, 3 June 2011, p. 1163; published online 2 December 2010) argued that the bacterial strain GFAJ-1 can vary the elemental composition of its biomolecules by substituting arsenic for phosphorus. Although their data show that GFAJ-1 is an extraordinary extremophile, consideration of arsenate redox chemistry undermines the suggestion that arsenate can replace the physiologic functions of phosphate.

The "green" phylogenetic clade of Rieske/cytb complexes.

More than a decade ago, Heliobacteria were recognised to contain a Rieske/cytb complex in which the cytochrome b subunit is split into two separate proteins, a peculiar feature characteristic of the cyanobacterial and plastidic b (6) f complex. The common presence of RCI-type reaction centres further emphasise possible evolutionary links between Heliobacteria, Chlorobiaceae and Cyanobacteria. In this contribution, we further explore the evolutionary relationships among these three phototrophic lineages by both molecular phylogeny and consideration of phylogenetic marker traits of the superfamily of Rieske/cytb complexes. The combination of these two methods suggests the existence of a "green" clade involving many non-phototrophs in addition to the mentioned RCI-type photosynthetic organisms. Structural and functional idiosyncrasies are (re-)interpreted in the framework of evolutionary biology and more specifically evolutionary bioenergetics.

Comment on "Arsenic (III) fuels anoxygenic photosynthesis in hot spring biofilms from Mono Lake, California".

Kulp et al. (Reports, 15 August 2008, p. 967) described a bacterium able to photosynthetically oxidize arsenite [As(III)] via arsenate [As(V)] reductase functioning in reverse. Based on their phylogenetic analysis of As(V) reductase, they proposed that this enzyme was responsible for the anaerobic oxidation of As(III) in the Archean. We challenge this proposition based on paleogeochemical, bioenergetic, and phylogenetic arguments.

Cytochromes c555 from the hyperthermophilic bacterium Aquifex aeolicus (VF5). 1. Characterization of two highly homologous, soluble and membranous, cytochromes c555.

Two distinct class I (monoheme) c-type cytochromes from the hyperthermophilic bacterium Aquifex aeolicus were studied by biochemical and biophysical methods (i.e., optical and EPR spectroscopy, electrochemistry). The sequences of these two heme proteins (encoded by the cycB1 and cycB2 genes) are close to identical (85% identity in the common part of the protein) apart from the presence of an N-terminal stretch of 62 amino acid residues present only in the cycB1 gene. A soluble cytochrome was purified and identified by N-terminal sequencing as the cycB2 gene product. It showed an alpha-peak at 555 nm, an E(m) value of +220 mV, and electron paramagnetic resonance parameters of gz = 2.89, gy = 2.287, and gx = 1.52. A firmly membrane-bound cytochrome characterized by nearly identical properties was detected and attributed to the cycB1 gene product. The very high degree of homology of its N-terminal part to cytochrome c553 from Heliobacterium gestii strongly suggests it to be anchored to the membrane via N-terminally attached lipid molecules. The two heme proteins were named cytochrome c555s (soluble) and cytochrome c555m (membranous). Electron paramagnetic resonance on partially ordered membrane multilayers suggests that the solvent-exposed heme domain of cytochrome c555m is flexible with respect to the membrane plane. Possible functional roles for both cytochromes are discussed.

The membrane-extrinsic domain of cytochrome b(558/566) from the archaeon Sulfolobus acidocaldarius performs pivoting movements with respect to the membrane surface.

The orientation of the membrane-attached cytochrome b(558/566)-haem with respect to the membrane was determined by electron paramagnetic resonance spectroscopy on two-dimensionally ordered oxidised membrane fragments from Sulfolobus acidocaldarius. Unlike the other redox centres in the membrane, the cytochrome b(558/566)-haem was found to cover a range of orientations between 25 degrees and 90 degrees. The described results are reminiscent of those obtained on the Rieske cluster of bc complexes and indicate that the membrane-extrinsic domain of cytochrome b(558/566) can perform pivoting motion between two extreme positions. Such a conformational flexibility is likely to play a role in electron transfer with its redox partners.