SummaryThe c-type cytochromes are haemoproteins that are subunits or physiological partners of electron transport chain components, like the cytochrome bc1 complex or the cbb3-type cytochrome c oxidase. Their haem moieties are covalently attached to the corresponding apocytochromes via a complex posttranslational maturation process. During our studies of cytochrome biogenesis, we uncovered a novel class of mutants that are unable to produce ornithine lipid and that lack several c-type cytochromes. Molecular analyses of these mutants led us to the ornithine lipid biosynthesis genes of Rhodobacter capsulatus. Herein, we have characterized these mutants, and established the chemical structure of this non-phosphorus membrane lipid from R. capsulatus. Ornithine lipids are known to induce potent host immune responses, including B-lymphocyte mitogenicity, adjuvanticity and macrophage activation. Yet, despite their widespread occurrence in Eubacteria, and the diverse biological effects they elicit in mammals, their physiological role in bacterial cells remained hitherto poorly defined. Our findings now indicate that under certain bacterial growth conditions ornithine lipids are crucial for optimal steady-state amounts of some extracytoplasmic proteins, including several c-type cytochromes, and attribute them a novel and important biological function.

We have recently established that the facultative phototrophic bacterium Rhodobacter sphaeroides, like the closely relatedRhodobacter capsulatus species, contains both the previously characterized mobile electron carrier cytochromec 2 (cyt c 2) and the more recently discovered membrane-anchored cytc y. However, R. sphaeroides cytc y, unlike that of R. capsulatus, is unable to function as an efficient electron carrier between the photochemical reaction center and the cyt bc 1complex during photosynthetic growth. Nonetheless, R. sphaeroides cyt c y can act at least in R. capsulatus as an electron carrier between the cytbc 1 complex and thecbb 3-type cyt c oxidase (cbb 3-Cox) to support respiratory growth. Since R. sphaeroides harbors both acbb 3-Cox and anaa 3-type cyt c oxidase (aa 3-Cox), we examined whetherR. sphaeroides cyt c y can act as an electron carrier to either or both of these respiratory terminal oxidases. R. sphaeroides mutants which lacked either cyt c 2 or cyt c y and either the aa 3-Cox or thecbb 3-Cox were obtained. These double mutants contained linear respiratory electron transport pathways between the cyt bc 1 complex and the cytc oxidases. They were characterized with respect to growth phenotypes, contents of a-, b-, andc-type cytochromes, cyt c oxidase activities, and kinetics of electron transfer mediated by cytc 2 or cyt c y. The findings demonstrated that both cyt c 2 and cytc y are able to carry electrons efficiently from the cyt bc 1 complex to either thecbb 3-Cox or theaa 3-Cox. Thus, no dedicated electron carrier for either of the cyt c oxidases is present in R. sphaeroides. However, under semiaerobic growth conditions, a larger portion of the electron flow out of the cytbc 1 complex appears to be mediated via the cytc 2-to-cbb 3-Coxand cytcy -to-cbb 3-Coxsubbranches. The presence of multiple electron carriers and cytc oxidases with different properties that can operate concurrently reveals that the respiratory electron transport pathways of R. sphaeroides are more complex than those ofR. capsulatus.

The cytochrome (cyt) c1 heme of the ubihydroquinone:cytochrome c oxidoreductase (bc1 complex) is covalently attached to two cysteine residues of the cyt c1 polypeptide chain via two thioether bonds, and the fifth and sixth axial ligands of its iron atom are histidine (H) and methionine (M), respectively. The latter residue is M183 in Rhodobacter capsulatus cyt c1, and previous mutagenesis studies revealed its critical role for the physicochemical properties of cyt c1 [Gray, K. A., Davidson, E., and Daldal, F. (1992) Biochemistry 31, 11864-11873]. In the homologous chloroplast b6f complex, the sixth axial ligand is provided by the amino group of the amino terminal tyrosine residue. To further pursue our investigation on the role played by the sixth axial ligand in heme-protein interactions, novel cyt c1 variants with histidine-lysine (K) and histidine-histidine axial coordination were sought. Using a R. capsulatus genetic system, the cyt c1 mutants M183K and M183H were constructed by site-directed mutagenesis, and chromatophore membranes as well as purified bc1 complexes obtained from these mutants were characterized in detail. The studies revealed that these mutants incorporated the heme group into the mature cyt c1 polypeptides, but yielded nonfunctional bc1 complexes with unusual spectroscopic and thermodynamic properties, including shifted optical absorption maxima (lambdamax) and decreased redox midpoint potential values (Em7). The availability and future detailed studies of these stable cyt c1 mutants should contribute to our understanding of how different factors influence the physicochemical and folding properties of membrane-bound c-type cytochromes in general.