Reactions of isocytochrome c2 in the photosynthetic electron transfer chain of Rhodobacter sphaeroides.

Rhodobacter sphaeroides strains lacking cytochrome c2 (cyt c2), the normal electron donor to P870+ in light-oxidized reaction center (RC) complexes, are unable to grow photosynthetically. However, spd mutations that suppress the photosynthetic deficiency of cyt c2 mutants elevate levels of the cyt c2 isoform, isocyt c2. We monitored photosynthetic electron transfer in whole cells, in chromatophores, and with purified components to ascertain if and how isocyt c2 reduced light-oxidized RC complexes. These studies revealed that several fundamental aspects of photosynthetic electron transfer were similar in strains that use isocyt c2 and wild-type cells. For example, P870+ reduction accompanied cytochrome c oxidation. In addition, photosynthetic electron transfer was blocked by the well-known cyt bc1 complex inhibitors antimycin and myxothiazol. However, even at the increased isocyt c2 levels present in these strains (approximately 40% that of cyt c2 in wild-type cells), there was little, if any, of the rapid (< 5 microns) electron transfer to P870+ that is characteristic of cytochromes bound to RC complexes at the time of the light flash. Thus, it appears that isocyt c2 function limits the in vivo rate of P870+ reduction. Indeed, at low ionic strength in vitro, the apparent affinity of isocyt c2 for RC complexes (KD approximately 40 microM) is significantly lower than that of cyt c2 (KD approximately 1.0 microM). This reduced affinity does not appear to result from an altered mode of RC binding by isocyt c2 since electrostatic interactions make similar overall contributions to the binding of both cyt c2 and isocyt c2 to this membrane-bound redox partner. Thus, sequence, structural, or local conformational differences between cyt c2 and isocyt c2 significantly alter their apparent affinities for this physiologically relevant redox partner.

Characterization of a glutathione-dependent formaldehyde dehydrogenase from Rhodobacter sphaeroides.

Glutathione-dependent formaldehyde dehydrogenases (GSH-FDH) represent a ubiquitous class of enzymes, found in both prokaryotes and eukaryotes. During the course of studying energy-generating pathways in the photosynthetic bacterium Rhodobacter sphaeroides, a gene (adhI) encoding a GSH-FDH homolog has been identified as part of an operon (adhI-cycI) that also encodes an isoform of the cytochrome c2 family of electron transport proteins (isocytochrome c2). Enzyme assays with crude Escherichia coli extracts expressing AdhI show that this protein has the characteristic substrate preference of a GSH-FDH. Ferguson plot analysis with zymograms suggests that the functional form of AdhI is a homodimer of approximately40-kDa subunits, analogous to other GSH-FDH enzymes. These properties of AdhI were used to show that mutations which increase or decrease adhI expression change the specific activity of GSH-FDH in R. sphaeroides extracts. In addition, expression of the presumed adhI-cycI operon appears to be transcriptionally regulated, since the abundance of the major adhI-specific primer extension product is increased by the trans-acting spd-7 mutation, which increases the level of both isocytochrome c2 and AdhI activity. While transcriptional linkage of adhI and cycI could suggest a function in a common metabolic pathway, isocytochrome c2 (periplasm) and AdhI (cytoplasm) are localized in separate compartments of R. sphaeroides. Potential roles for AdhI in carbon and energy generation and the possible relationship of GSH-FDH activity to isocytochrome c2 will be discussed based on the commonly accepted physiological functions of GSH-FDH enzymes in prokaryotes and eukaryotes.

Genetic evidence for the role of isocytochrome c2 in photosynthetic growth of Rhodobacter sphaeroides Spd mutants.

In Rhodobacter sphaeroides, cytochrome c2 (cyt c2)-deficient mutants are photosynthetically incompetent (PS-). However, mutations which suppress the photosynthetic deficiency (spd mutations) of cyt c2 mutants increase the levels of a cyt c2 isoform, isocyt c2. To determine whether isocyt c2 was required for photosynthetic growth of Spd mutants, we used Tn5 mutagenesis to generate a PS- mutant (TP39) that lacks both cyt c2 and isocyt c2. DNA sequence analysis of wild-type DNA that restores isocyt c2 production and photosynthetic growth to TP39 indicates that it encodes the isocyt c2 structural gene, cycI. The Tn5 insertion in TP39 is approximately 1.5 kb upstream of cycI, and our results show that it is polar onto cycI. The cycI gene has been physically mapped to a region of chromosome I that is approximately 700 kb from the R. sphaeroides photosynthetic gene cluster. Construction of a defined cycI null mutant and complementation of several mutants with the cycI gene under the control of the cyt c2 promoter region indicate that an increase in the levels of isocyt c2 alone is necessary and sufficient for photosynthetic growth in the absence of cyt c2. The data are discussed in terms of the obligate role of isocyt c2 in cyt c2-independent photosynthesis of R. sphaeroides.

Regulation of a cytochrome c2 isoform in wild-type and cytochrome c2 mutant strains of Rhodobacter sphaeroides.

In Rhodobacter sphaeroides, mutations that suppress the photosynthetic deficiency (spd mutations) of strains lacking cytochrome c2 (cyt c2) cause accumulation of a periplasmic cyt c2 isoform that has been designated isocytochrome c2 (isocyt c2). In this study, a new method for purification of both cyt c2 and isocyt c2 is described that uses periplasmic fluid as a starting material. In addition, antiserum to isocyt c2 has been used to demonstrate that all suppressor mutants contain an isocyt c2 of approximately 15 kDa. Western blot analysis indicates that isocyt c2 was present at lower levels in both wild-type and cyt c2 mutants than in spd-containing mutants. Although isocyt c2 is detectable under all growth conditions in wild-type cells, the highest level of isocyt c2 is present under aerobic conditions. Our results demonstrate that spd mutations increase the steady state level of isocyt c2 under photosynthetic conditions. Although the physiological function of isocyt c2 in wild-type cells is not known, we show that a nitrate-regulated protein in Rhodobacter sphaeroides f. sp. denitrificans also reacts with the isocyt c2 antiserum.

Rhodobacter sphaeroides spd mutations allow cytochrome c2-independent photosynthetic growth.

In Rhodobacter sphaeroides, cytochrome c2 (cyt c2) is a periplasmic redox protein required for photosynthetic electron transfer. cyt c2-deficient mutants created by replacing the gene encoding the apoprotein for cyt c2 (cycA) with a kanamycin resistance cartridge are photosynthetically incompetent. Spontaneous mutations that suppress this photosynthesis deficiency (spd mutants) arise at a frequency of 1 to 10 in 10(7). We analyzed the cytochrome content of several spd mutants spectroscopically and by heme peroxidase assays. These suppressors lacked detectable cyt c2, but they contained a new soluble cytochrome which was designated isocytochrome c2 (isocyt c2) that was not detectable in either cycA+ or cycA mutant cells. When spd mutants were grown photosynthetically, isocyt c2 was present at approximately 20 to 40% of the level of cyt c2 found in photosynthetically grown wild type cells, and it was found in the periplasm with cytochromes c' and c554. These spd mutants also had several other pleiotropic phenotypes. Although photosynthetic growth rates of the spd mutants were comparable to those of wild-type strains at all light intensities tested, they contained elevated levels of B800-850 pigment-protein complexes. Several spd mutants contained detectable amounts of isocyt c2 under aerobic conditions. Finally, heme peroxidase assays indicated that, under anaerobic conditions, the spd mutants may contain another new cytochrome in addition to isocyt c2. These pleiotropic phenotypes, the frequency at which the spd mutants arise, and the fact that a frameshift mutagen is very effective in generating the spd phenotype suggest that some spd mutants contain a mutation in loci which regulate cytochrome synthesis.

Expression of a cytochrome c2 isozyme restores photosynthetic growth of Rhodobacter sphaeroides mutants lacking the wild-type cytochrome c2 gene.

Deletion of the cytochrome c2 gene in the purple bacterium Rhodobacter sphaeroides renders it incapable of phototrophic growth (strain cycA65). However, suppressor mutants which restore the ability to grow phototrophically are obtained at relatively high frequency (1-10 in 10(7)). We examined two such suppressors (strains cycA65R5 and cycA65R7) and found the expected complement of electron transfer proteins minus cytochrome c2: SHP, c', c551.5, and c554. Instead of cytochrome c2 which elutes from DEAE-cellulose between SHP and cytochrome c', at about 50 mM ionic strength in wild-type extracts, we found a new high redox potential cytochrome c in the mutants which elutes with cytochrome c551.5 at about 150 mM ionic strength. The new cytochrome is more acidic than cytochrome c2, but is about the same size or slightly smaller (13,500 Da). The redox potential of the new cytochrome from strain cycA65R7 (294 mV) is about 70 mV lower than that of cytochrome c2. The 280 nm absorbance of the new cytochrome is smaller than that of cytochrome c2, which suggests that there is less tryptophan (the latter has two residues). In vitro kinetics of reduction by lumiflavin and FMN semiquinones show that the reactivity of the new cytochrome is similar to that of cytochrome c2, and that there is a relatively large positive charge (+2.6) at the site of reduction, despite the overall negative charge of the protein. This behavior is characteristic of cytochromes c2 and unlike the majority of bacterial cytochromes examined. Fourteen out of twenty-four of the N-terminal amino acids of the new cytochrome are identical to the sequence of cytochrome c2. The N-termini of the cycA65R5 and cycA65R7 cytochromes were the same. The kinetics and sequence data indicate that the new protein may be a cytochrome c2 isozyme, which is not detectable in wild-type cells under photosynthetic growth conditions. We propose the name iso-2 cytochrome c2 for the new cytochrome produced in the suppressor strains.