PetC1 is the major Rieske iron-sulfur protein in the cytochrome b6f complex of Synechocystis sp. PCC 6803.

Many of the completely sequenced cyanobacterial genomes contain a gene family that encodes for putative Rieske iron-sulfur proteins. The Rieske protein is one of the large subunits of the cytochrome bc-type complexes involved in respiratory and photosynthetic electron transfer. In contrast to all other subunits of this complex that are encoded by single genes, the genome of the cyanobacterium Synechocystis PCC 6803 contains three petC genes, all encoding potential Rieske subunits. Most interestingly, any of the petC genes can be deleted individually without altering the Synechocystis phenotype dramatically. In contrast, double deletion experiments revealed that petC1 and petC2 cannot be deleted in combination, whereas petC3 can be deleted together with any of the other two petC genes. Further results suggest a different physiological function for each of the Rieske proteins. Whereas PetC2 can partly replace the dominating Rieske isoform PetC1, PetC3 is unable to functionally replace either PetC1 or PetC2 and may have a special function involving a special donor with a lower redox potential than plastoquinone. A predominant role of PetC1, which is (partly) different from PetC2, is suggested by the mutational analysis and a detailed characterization of the electron transfer reactions in the mutant strains.

Endosymbiosis and the design of eukaryotic electron transport.

The bioenergetic organelles of eukaryotic cells, mitochondria and chloroplasts, are derived from endosymbiotic bacteria. Their electron transport chains (ETCs) resemble those of free-living bacteria, but were tailored for energy transformation within the host cell. Parallel evolutionary processes in mitochondria and chloroplasts include reductive as well as expansive events: On one hand, bacterial complexes were lost in eukaryotes with a concomitant loss of metabolic flexibility. On the other hand, new subunits have been added to the remaining bacterial complexes, new complexes have been introduced, and elaborate folding patterns of the thylakoid and mitochondrial inner membranes have emerged. Some bacterial pathways were reinvented independently by eukaryotes, such as parallel routes for quinol oxidation or the use of various anaerobic electron acceptors. Multicellular organization and ontogenetic cycles in eukaryotes gave rise to further modifications of the bioenergetic organelles. Besides mitochondria and chloroplasts, eukaryotes have ETCs in other membranes, such as the plasma membrane (PM) redox system, or the cytochrome P450 (CYP) system. These systems have fewer complexes and simpler branching patterns than those in energy-transforming organelles, and they are often adapted to non-bioenergetic functions such as detoxification or cellular defense.

Potassium uptake in the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 mainly depends on a Ktr-like system encoded by slr1509 (ntpJ).

The molecular basis of potassium uptake in cyanobacteria has not been elucidated. However, genes known from other bacteria to encode potassium transporters can be identified in the genome of Synechocystis sp. strain PCC 6803. Mutants defective in kdpA and ntpJ were generated and characterized to address the role of the Kdp and KtrAB systems in this strain. KtrAB is crucial for K(+) uptake, as the DeltantpJ mutant shows slowed growth, slowed potassium uptake kinetics, and increased salt sensitivity. The DeltakdpA mutant has the same phenotype as the wild type even at limiting potassium, but a DeltakdpADeltantpJ double mutant is not viable, indicating a role of Kdp for potassium uptake when the Ktr system is not functioning.

Adaptation to iron deficiency: a comparison between the cyanobacterium Synechococcus elongatus PCC 7942 wild-type and a DpsA-free mutant.

To learn more about the adaptive response of Synechococcus elongatus PCC 7942 to iron starvation and the role of DpsA, presumably a protein protecting chromosomal DNA against oxidative damage, we performed a comparative analysis of S. elongatus PCC 7942 wild-type and a DpsA-free mutant, called K11. Relative to wild-type, the DpsA-free mutant had significantly higher amounts of phycocyanin and allophycocyanin, even upon iron limitation. While the Photosystem I activity in mutant K11 remained high under iron deficiency, the Photosystem II activity dropped severely with respect to wild-type. The DpsA content in wild-type was already fairly high under regular growth conditions and did not significantly increase under iron deficiency nor in the presence of 0.3 mM 2'2'-dipyridyl in iron-sufficient BG11 medium. Nevertheless, the absence of DpsA in K11 resulted in a significantly altered transcriptional/translational activity of genes known to be involved in adaptation to iron starvation. The amount of isiA/B transcript was about two-fold lower than in wild-type, resulting in a lower 77 K chlorophyll a fluorescence at 685 nm, implying a lower concentration of Photosystem I-IsiA supercomplexes. While in wild-type idiA, idiB, and irpA transcripts were highly up-regulated, hardly any were detectable in mutant K11 under iron limitation. The concentration of mapA transcript, however, was greatly increased in K11 compared to wild-type. Measurements of acridine yellow fluorescence with intact wild-type and K11 cells revealed that iron deficiency caused an increased contribution of cyclic electron transport to membrane energisation and ATP synthesis being in agreement with the formation of the Photosystem I-IsiA supercomplex. In addition, mutant K11 had a much higher respiratory activity compared to wild-type under iron limitation.

The chemical basis of membrane bioenergetics.

All organisms rely on chemiosmotic membrane systems for energy transduction; the great variety of participating proteins and pathways can be reduced to a few universal principles of operation. This chemical basis of bioenergetics is reviewed with respect to the origin and early evolution of life. For several of the cofactors which play important roles in bioenergetic reactions, plausible prebiotic sources have been proposed, and it seems likely that these cofactors were present before elaborate protein structures. In particular, the hydrophobic quinones require only a membrane-enclosed compartment to yield a minimum chemiosmotic system, since they can couple electron transport and proton translocation in a simple way. It is argued that the central features of modern bioenergetics, such as the coupling of redox reactions and ion translocation at the cytoplasmic membrane, probably are ancient features which arose early during the process of biogenesis. The notion of a thermophile root of the universal phylogenetic tree has been discussed controversially, nevertheless, thermophiles are interesting model organisms for reconstructing the origin of chemiosmotic systems, since they are often acidophiles and anaerobic respirers exploiting iron-sulfur chemistry. This perspective can help to explain the prominent role of iron-sulfur proteins in extant biochemistry as well as the origin of both respiration and proton extrusion within the context of a possible origin of life in the vicinity of hot vents.

Electron transport routes in whole cells of Synechocystis sp. strain PCC 6803: the role of the cytochrome bd-type oxidase.

The plastoquinone pool is the central switching point of both respiratory and photosynthetic electron transport in cyanobacteria. Its redox state can be monitored noninvasively in whole cells using chlorophyll fluorescence induction, avoiding possible artifacts associated with thylakoid membrane preparations. This method was applied to cells of Synechocystis sp. PCC 6803 to study respiratory reactions involving the plastoquinone pool. The role of the respiratory oxidases known from the genomic sequence of Synechocystis sp. PCC 6803 was investigated by a combined strategy using inhibitors and deletion strains that lack one or more of these oxidases. The putative quinol oxidase of the cytochrome bd-type was shown to participate in electron transport in thylakoid membranes. The activity of this enzyme in thylakoids was strongly dependent on culture conditions; it was increased under conditions where the activity of the cytochrome b(6)f complex alone may be insufficient for preventing over-reduction of the PQ pool. In contrast, no indication of quinol oxidase activity in thylakoids was found for a second alternative oxidase encoded by the ctaII genes.