We describe the construction and characterization of Essentially three features have made the gram-negative soil bacterium Paracoccus denitrificans an interesting organism for bioenergetic research and, in particular, a model organism for the mitochondrial electron transport. (i) Its close relationship to present-day mitochondria, indicated by a large number of mostly physiological and bioenergetic arguments, prompted John and Whatley (23,24) to assign this bacterium a hypothetical precursor role in endosymbiotic organelle development; this early assumption of a close evolutionary homology has recently been confirmed by more direct evidence on phylogenetic grounds (59). (ii) At least two respiratory complexes, cytochrome c reductase (see below) and cytochrome c oxidase (17,30,32) isolated from this bacterium in a functional state, have basic enzymatic properties almost indistinguishable from those of their mitochondrial counterparts, but are far less complex in their structure. (iii) Due to the availability of the genes for their subunits, these enzymes are amenable to site-specific mutagenesis experiments to study stucture-function relationships and thus increase our knowledge on basic electron transport and energy transduction mechanisms both in bacteria and in mitochondria.The cytochrome bc, complex (complex III; ubiquinol: cytochrome c oxidoreductase [EC 1.10.2.2]) is an obligate component in the electron transport chain of mitochondria and, in an analogous form, of photosynthetic organisms (19,20,43,58). Moreover, several bacterial species have recently been shown to contain a functionally homologous complex, notably Rhodobacter species (8,13,14) and Paracoccus denitrificans (4, 33, 60). Three different subunits carrying the redox centers of this integral membrane enzyme, cytochromes b and cl and the so-called Rieske protein with its iron-sulfur center (FeS), are common to all bc, complexes. Whereas the well-studied enzyme complexes from mitochondria typically contain a large number of additional polypeptides of uncertain function, the bacterial enzymes mentioned above are composed of only the three essential redox center subunits. However, at least for Para-* Corresponding author. coccus spp., full enzymatic competence of this structurally simple complex has been reported in a reconstituted system, for both its electron transport and energy transduction capacity (60, 61). Molecular details of these two coupled processes have not been elucidated in mitochondrial bc1 complexes, but hypotheses for structure-function relationships as well as tentative functional assignments for certain residues have been made (10,22,38,45,52,57). Such assumptions can be tested in bacteria by performing sitedirected mutagenesis on the isolated genes, followed by insertion into a suitable host to obtain expression of the redox complex. In Paracoccus spp., the genes for the bc1 subunits have been cloned and sequenced (27), and, as in Rhodobacter spp. (8, 13), have been shown to be organized in an operon structure, subsequent...

No abstract

Data are presented on three components of the quinol oxidation branch of the Paracoccus respiratory chain: cytochrome c reductase, cytochrome c552, and the a-type terminal oxidase. Deletion mutants in the bc1 and the aa3 complex give insight into electron pathways, assembly processes, and stability of both redox complexes, and, moreover, are an important prerequisite for future site-directed mutagenesis experiments. In addition, evidence for a role of cytochrome c552 in electron transport between complex III and IV is presented.

The gene encoding the Rhodopseudomonas viridis cytochrome c2 (cycA) has been introduced on a broad host range vector into Paracoccus denitrificans, leading to high-level expression of the holo-cytochrome with the heme moiety covalently attached to the apoprotein. The cytochrome was demonstrated to reside in the periplasmic space of the host cell. In contrast to R. viridis, aerobic rather than anaerobic growth conditions led to higher production levels of the holo-cytochrome in P. denitrificans. This heterologous expression system provides a suitable genetic background for the functional expression and mutagenesis of polypeptides involved in bacterial photosynthesis, offering the possibility of detailed structural and functional investigation.

No abstract