The iron respiratory chain of the acidophilic bacterium Acidithiobacillus ferrooxidans involves various metalloenzymes. Here we demonstrate that the oxygen reduction pathway from ferrous iron (named downhill pathway) is organized as a supercomplex constituted of proteins located in the outer and inner membranes as well as in the periplasm. For the first time, the outer membrane-bound cytochrome c Cyc2 was purified, and we showed that it is responsible for iron oxidation and determined that its redox potential is the highest measured to date for a cytochrome c. The organization of metalloproteins inside the supramolecular structure was specified by protein-protein interaction experiments. The isolated complex spanning the two membranes had iron oxidase as well as oxygen reductase activities, indicating functional electron transfer between the first iron electron acceptor, Cyc2, and the Cu A center of cytochrome c oxidase aa 3 . This is the first characterization of a respirasome from an acidophilic bacterium. In Acidithiobacillus ferrooxidans, O 2 reduction from ferrous iron must be coupled to the energy-consuming reduction of NAD ؉ (P) from ferrous iron (uphill pathway) required for CO 2 fixation and other anabolic processes. Besides the proteins involved in the O 2 reduction, there were additional proteins in the supercomplex, involved in uphill pathway (bc complex and cytochrome Cyc 42 ), suggesting a possible physical link between these two pathways.

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Enterococcus faecalis cells cannot synthesize porphyrins and do not rely on heme for growth but can take up heme and use it to synthesize heme proteins. We recently described a cytochrome bd in E. faecalis strain V583 and here report the identification of a chromosomal gene, katA, encoding a heme-containing cytoplasmic catalase. The 54-kDa KatA polypeptide shows sequence similarity to members of the family of monofunctional catalases. A hexahistidyl-tagged version of the catalase was purified, and major characteristics of the enzyme were determined. It contains one protoheme IX group per KatA polypeptide. Catalase activity was detected only in E. faecalis cells grown in the presence of heme in the medium; about 2 and 10 M hemin was required for half-maximal and maximal production of catalase, respectively. Our finding of a catalase whose synthesis is dependent on the acquisition of heme in the opportunistic pathogen E. faecalis might be of clinical importance. Studies of cellular heme transport and heme protein assembly and in vivo synthesis of metalloprotein analogs for biotechnological applications are impeded by the lack of experimental systems. We conclude that the E. faecalis cell potentially provides such a desired system.Enterococcus faecalis, formerly known as Streptococcus faecalis, is a gram-positive bacterium with a low GϩC content in its genomic DNA. It is a common inhabitant of the intestines of animals, including humans, where it is part of the commensal flora. However, E. faecalis can cause disease in, e.g., immunodeficient persons, and it is frequently the causative agent of nosocomial infections. Lately, the emergence of multidrugresistant strains has become a serious problem (10). Recognizing the medical importance of this bacterium, The Institute for Genomic Research has undertaken the genome sequencing of E. faecalis strain V583.Heme is present as a prosthetic group in a variety of proteins, including, for example, catalases, cytochromes, and hemoglobins. E. faecalis cells do not synthesize heme, and genes for known porphyrin biosynthetic enzymes are not found in the genome sequence of strain V583. However, if E. faecalis cells are supplied with heme, synthesis of hemoproteins can take place (16,19). We recently described a functional cytochrome bd-type quinol oxidase in E. faecalis V583 (27). The cytochrome bd is probably the terminal oxidase of a respiratory chain present in E. faecalis under certain conditions (1, 15). The presence of an aerobic respiratory chain is puzzling, however, in a bacterium generally considered to use a fermentative-energy metabolism. Aerobic respiration is more energyefficient than fermentation, but it is also a source of reactive oxygen species. Cells have several protective mechanisms against these toxic compounds. Enzymatic detoxification of hydrogen peroxide in bacteria is mainly performed by catalases. Three classes of bacterial catalases have been described: monofunctional catalases, catalase-peroxidases, and manganese catalases (pseudocatalases) (30). The ...

The reacton centre I (RCI)-type photosystems from plants, cyano-, helio- and green sulphur bacteria are compared and the essential properties of an archetypal RCI are deduced. Species containing RCI-type photosystems most probably cluster together on a common branch of the phylogenetic tree. The predicted branching order is green sulphur, helio- and cyanobacteria. Striking similarities between RCI- and RCII-type photosystems recently became apparent in the three-dimensional structures of photosystem I (PSI), PSII and RCII. The phylogenetic relationship between all presently known photosystems is analysed suggesting (a) RCI as the ancestral photosystem and (b) the descendence of PSII from RCI via gene duplication and gene splitting. An evolutionary model trying to rationalise available data is presented.