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Query: UMLS:C1832526 (PCC)
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Photoautotrophically grown cyanobacterium Nostoc sp. strain Mac (PCC 8009) released up to about 10 nmol of a c-type cytochrome per ml packed cells after treatment with EDTA under conditions that left the plasma membrane absolutely intact as judged from the absence of cytosolic proteins in the supernatant. Spectra of the ascorbate reduced cytochrome revealed peaks at 553, 522 and 416 nm. The protein was purified to an A-553/A-275 ratio of 0.8. Midpoint potential (at pH 7), isoelectric point and apparent molecular weight of the cytochrome were +0.35 V, 8.6, and around 10,500, respectively. The cytochrome proved to be an excellent electron donor to the aa3-type cytochrome oxidase in both plasma and thylakoid membranes isolated and purified from Nostoc Mac. Chemoheterotrophic growth of the cells increased the level of periplasmic cytochrome c up to 10-fold and cytochrome oxidase activity of plasma membranes up to 90-fold. The periplasmic cytochrome also transferred electrons to photosystem I in illuminated thylakoid membranes. We conclude that cyanobacteria contain a periplasmic c-type cytochrome presumably identical to so-called cytochrome c6 or c-553 which has long been known as a photosynthetic (i.e. thylakoid-associated) redox protein in these organisms, and which is capable of donating electrons (from the periplasmic space) to the cytochrome oxidase in the plasma membrane and (from the thylakoid lumen) to both P700 and cytochrome oxidase in the thylakoid membrane.
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PMID:Identification of a periplasmic C-type cytochrome as electron donor to the plasma membrane-bound cytochrome oxidase of the cyanobacterium Nostoc Mac. 216 67

The thylakoid membrane cytochrome b6-f complex (plastoquinol:oxidized-plastocyanin oxidoreductase, EC 1.10.99.1) catalyzes electron-transfer and proton-translocation reactions essential for oxygenic photosynthesis. We have isolated and determined the nucleotide sequences of the petC and petA genes encoding the Rieske Fe-S and cytochrome f polypeptides from the filamentous cyanobacterium Nostoc PCC 7906. These genes occur as single genomic copies, are tightly linked, and, as indicated by hybridization of gene-specific probes to Nostoc RNA, are cotranscribed as a 2.0-kilobase message. The Rieske Fe-S/cytochrome f gene pair thus represents an example of clustering and cotranscription in cyanobacteria of functionally related genes that, in photosynthetic eukaryotes, reside on separate nuclear and plastid genomes. These data are consistent with the progressive degeneration of the modern chloroplast genome from the ancestral, cyanobacterial-like genome of an endosymbiont. The Rieske Fe-S and the mature cytochrome f apoproteins are encoded by 537 and 867 nucleotides and have molecular masses of 19.2 and 31.2 kDa, respectively. They show 59% and 60% protein sequence identity, respectively, relative to spinach. Forty-four amino acids (4.7 kDa) resembling a prokaryotic signal sequence precede apocytochrome f. In contrast, the Rieske Fe-S protein appears to be translated without a presequence. The 183 bases separating the Rieske Fe-S and preapocytochrome f genes contain two families of 7- to 9-base tandem repeats, and some part of this sequence is highly reiterated in the genome. The C terminus of the Rieske Fe-S protein contains cysteine and histidine residues (probable ligands for the Fe2S2 center) in two peptides, Cys-Thr-His-Leu-Gly-Cys-Val and Cys-Pro-Cys-His-Gly-Ser, which have been conserved in spinach and in the five available Rieske Fe-S sequences from the mitochondrial-type cytochrome b-c1 complexes. Cytochrome f shows the heme binding residues Cys-Xaa-Xaa-Cys-His near its N terminus. Single, long hydrophobic stretches occur near the N and C termini, respectively, of the Rieske Fe-S and cytochrome f proteins and may form membrane-spanning helices.
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PMID:Primary structure of cotranscribed genes encoding the Rieske Fe-S and cytochrome f proteins of the cyanobacterium Nostoc PCC 7906. 284 48

We have isolated and determined the nucleotide and derived protein sequences for the four genes, petCA and BD, which encode the cytochrome b6-f, electron-transfer complex of the filamentous cyanobacterium, Nostoc PCC 7906. The primary structure and cotranscription of the petCA genes encoding the Rieske-FeS (nuclear encoded in plants) and apocytochrome f proteins has been described previously (Kallas, T., Spiller, S., and Malkin, R. (1988) Proc. Natl. Acad. Sci. U.S.A., in press). The petBD genes (645 and 480 protein-coding nucleotides, respectively) for the apocytochrome b6 (24.3 kDa) and subunit-IV (17.5 kDa) proteins comprise a second operon located at least 12 kilobases (kb) from petCA. The Nostoc petBD genes are not closely linked to the psbB gene (encoding the 51-kDa photosystem II polypeptide) and do not contain introns as do the closely related chloroplast genes. DNA probes specific for each of the Nostoc cytochrome-complex genes hybridized to single bands in genomic DNA blots at intensities expected for single copy genes. These data suggest that a single set of cytochrome b6-f proteins function in the different types of membranes found in Nostoc vegetative and heterocyst cells. RNA blot hybridizations identified an 1.8-kb mRNA common to cytochrome b6 and subunit IV, and an intensely hybridizing 0.8-kb mRNA specific to the subunit IV gene probe. The role of the latter RNA is not clear but it may represent a transcript from the opposite strand. The deduced Rieske, apocytochrome f, apocytochrome b6, and subunit IV proteins exhibit 59, 58-63, 84-85, and 79-83% sequence identity with the proteins from chloroplast cytochrome b6-f complexes. The Nostoc proteins show lower but still significant sequences identity with the corresponding proteins of the mitochondrial-type b-c1 complexes. The four probable heme-liganding His residues, and the approximate spacings between them, have been conserved in all of the available cytochrome b6 and b sequences from divergent sources. The Nostoc apocytochrome b6 and subunit IV proteins, as well as the Rieske, appear to be translated and thus inserted into the membrane as mature forms without cleavable presequences. Hydropathy analyses revealed five potential membrane spans in cytochrome b6 and three in the subunit IV protein, consistent with the profiles observed for the chloroplast proteins and the related cytochrome b proteins of cytochrome b-c1 complexes.
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PMID:Characterization of two operons encoding the cytochrome b6-f complex of the cyanobacterium Nostoc PCC 7906. Highly conserved sequences but different gene organization than in chloroplasts. 284 67

Photosystem I catalyzes the light-driven oxidation of plastocyanin or cytochrome c6 and the reduction of ferredoxin or flavodoxin. PsaJ is a 4.4 kDa hydrophobic subunit of photosystem I from cyanobacteria and chloroplasts. To investigate the function of PsaJ, we generated a mutant strain of the cyanobacterium Synechocystis sp. PCC 6803 in which the psaJ gene is replaced by a gene for chloramphenicol resistance. Deletion of psaJ led to a reduction in the steady state RNA level from psaF which is located upstream from psaJ. Immunoquantification using an anti-PsaF antibody revealed a significant decrease in the amount of PsaF in membranes of the mutant strain. Trimeric photosystem I complexes isolated from the mutant strain using n-dodecyl beta-D-maltoside lacked PsaJ, contained ca. 80% less PsaF, but maintained wild-type levels of other photosystem I subunits. In contrast, the photosystem I purified using Triton X-100 contained less than 2% PsaF when compared to the wild type, showing the more extractable nature of PsaF in PsaJ-less photosystem I in the presence of Triton X-100. PsaE was more accessible to removal by NaI in a mutant strain lacking PsaF and PsaJ than in the wild type. The presence of PsaF in photosystem I from the PsaJ-less strain did not alter the increased susceptibility of PsaE to removal by NaI. These results indicate an interaction between PsaJ and PsaF in the organization of the complex.
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PMID:Targeted deletion of psaJ from the cyanobacterium Synechocystis sp. PCC 6803 indicates structural interactions between the PsaJ and PsaF subunits of photosystem I. 752 26

The reaction mechanism of electron transfer from the interchangeable metalloproteins plastocyanin (Pc) and cytochrome c6 (Cyt) to photooxidized P700 in photosystem I (PSI) has been studied by laser-flash absorption spectroscopy using a number of evolutionarily differentiated organisms such as cyanobacteria (Anabaena sp. PCC 7119 and Synechocystis sp. PCC 6803), green algae (Monoraphidium braunii), and higher plants (spinach). PSI reduction by Pc or Cyt shows different kinetics depending on the organism from which the photosystem and metalloproteins are isolated. According to the experimental data herein reported, three different kinetic models are proposed by assuming either an oriented collisional reaction mechanism (type I), a minimal two-step mechanism involving complex formation followed by intracomplex electron transfer (type II), or rearrangement of the reaction partners within the complex before electron transfer takes place (type III). Our findings suggest that PSI was able to first optimize its interaction with positively charged Cyt and that the evolutionary replacement of the ancestral Cyt by Pc, as well as the appearance of the fast kinetic phase in the Pc/PSI system of higher plants, would involve structural modifications in both the donor protein and PSI.
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PMID:Laser-flash kinetic analysis of the fast electron transfer from plastocyanin and cytochrome c6 to photosystem I. Experimental evidence on the evolution of the reaction mechanism. 754 59

Extraction and identification of the non-covalently bound heme groups from crude membrane preparations of photoheterotrophically grown Synechocystis sp. PCC 6803 by reversed phase high performance liquid chromatography and optical spectrophotometry led to the detection of heme O in addition to hemes B and A which latter was to be expected from the known presence of aa3-type cytochrome oxidase in cyanobacteria. In fully aerated cells (245 microM dissolved O2 in the medium) besides heme B only heme A was found while in low-oxygen cells (< 10 microM dissolved O2) heme O was present at a concentration even higher than that of heme A. Given the possible role of heme O as a biosynthetic intermediate between heme B and heme A, together with generally much higher Km values of 5-50 microM O2 for oxygenase as compared to Km values of 40-70 nM O2 for typical cytochrome-c oxidase, our findings may permit the conclusion that the conversion of heme O to heme A is an obligately oxygen-requiring process catalyzed by some oxygenase directly introducing oxygen from O2 into the 8-methyl group of heme O. At the same time thus the occurrence of heme O (cytochrome o) in cyanobacteria does of course not imply the existence of an 'alternative oxidase' since according to the well-known 'promiscuity of heme groups' both hemes O and A are likely to combine with one and the same apoprotein.
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PMID:Occurrence of heme O in photoheterotrophically growing, semi-anaerobic cyanobacterium Synechocystis sp. PCC6803. 767 30

A soluble low-potential cytochrome c549 has been purified in milligram quantities from the cyanobacterium Synechocystis sp. PCC 6803. The protein exhibits an acid isoelectric point of 3.9, a molecular mass of 15.8 kDa, and a midpoint redox potential value of -250 mV at pH 7.0 EPR and 1H NMR studies suggest a low-spin heme iron with bis-histidine coordination at the fifth and sixth positions. EDTA-photoreduced 5-deazariboflavin has been used as the electron-donating system to study, by laser flash absorption spectroscopy, the electron transfer reactions between Synechocystis cytochrome c549 and redox proteins involved in the cyclic electron flow around photosystem I. The second-order rate constants (k2) obtained for ferredoxin (or flavodoxin) oxidation by Synechocystis cytochrome c549 are rather low (ca. 10(5) M-1 s-1), thus suggesting that this low-potential heme-protein does not operate as the primary electron carrier for either transferring electrons to the cytochrome b6f complex in cyclic photophosphorylation or to hydrogenase during anaerobic metabolism. The k2 values for plastocyanin reduction by cytochrome c549 are about 100 times higher (ca. 10(7) M-1 s-1), but it remains to be determined whether or not this reaction actually reflects a physiological process.
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PMID:Purification and physicochemical properties of the low-potential cytochrome C549 from the cyanobacterium Synechocystis sp. PCC 6803. 772 71

The gene encoding the cytochrome b6 subunit (petB) of the cytochrome b6f complex has been isolated, cloned and sequenced by nonradioactive methods from genomic DNA of the unicellular cyanobacterium Synechocystis sp. PCC 6803. The coding region consists of 666 nucleotides, coding for a polypeptide with a molecular mass of 25.02 kDa. In contrast to higher plant petB sequences an aminoterminal extension of seven amino acids occurs. Aminoterminal sequencing of the isolated protein excludes--different from higher plants--the existence of an intron after the first amino acids but indicate the posttranslational removal of three amino acids from the amino terminus. The aminoterminal extension--found only in non-nitrogen-fixing, unicellular cyanobacteria--shows a high degree of homology between different species.
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PMID:Nucleotide sequence of the petB gene encoding cytochrome b6 from the mesophilic cyanobacterium Synechocystis PCC 6803: implications for evolution and function. 780 59

The petE gene encoding plastocyanin precursor protein from the cyanobacterium Anabaena PCC 7937 was introduced in the cyanobacterial host strain Synechococcus PCC 7942. The host normally only uses cytochrome c553 as Photosystem I (PS I) donor. The heterologous gene was efficiently expressed using the inducible Escherichia coli trc promoter. Accumulation of plastocyanin protein depended on the presence of Cu2+. The protein was accurately targeted to the thylakoid lumen, from which it could be isolated in the mature form. Redox difference spectroscopy proved the presence of a Cu2+ ion in the holoenzyme. Isolated heterologous plastocyanin was functional in reconstitution of in vitro electron transfer to PS I. The presence of Anabaena plastocyanin in Synechococcus thylakoid membranes increased PS I electron transfer rate 2.5 times. Analysis of P700 redox and PS II fluorescence transients in vivo showed a faster electron transfer through PS I because of enhanced electron supply in the presence of plastocyanin. In addition, the distribution of electrons between photosynthetic and respiratory electron transfer changed. Plastocyanin preferentially donates electrons to PS I rather than to the respiratory cytochrome-c oxidase complex and is not functionally equivalent to cytochrome c553.
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PMID:Expression of Anabaena PCC 7937 plastocyanin in Synechococcus PCC 7942 enhances photosynthetic electron transfer and alters the electron distribution between photosystem I and cytochrome-c oxidase. 796 43

Photosystem I (PSI) is a multisubunit enzyme that catalyzes the light-driven oxidation of plastocyanin or cytochrome c6 and the concomitant photoreduction of ferredoxin or flavodoxin. To identify the surface-exposed domains in PSI of the cyanobacterium Synechocystis sp. PCC 6803, we mapped the regions in PsaE, PsaD, and PsaF that are accessible to proteases and N-hydroxysuccinimidobiotin (NHS-biotin). Upon exposure of PSI complexes to a low concentration of endoproteinase glutamic acid (Glu)-C, PsaE was cleaved to 7.1- and 6.6-kD N-terminal fragments without significant cleavage of other subunits. Glu63 and Glu67, located near the C terminus of PsaE, were the most likely cleavage sites. At higher protease concentrations, the PsaE fragments were further cleaved and an N-terminal 9.8-kD PsaD fragment accumulated, demonstrating the accessibility of Glu residue(s) in the C-terminal domain of PsaD to the protease. Besides these major, primary cleavage products, several secondary cleavage sites on PsaD, PsaE, and PsaF were also identified. PsaF resisted proteolysis when PsaD and PsaE were intact. Glu88 and Glu124 of PsaF became susceptible to endoproteinase Glu-C upon extensive cleavage of PsaD and PsaE. Modification of PSI proteins with NHS-biotin and subsequent cleavage by endoproteinase Glu-C or thermolysin showed that the intact PsaE and PsaD, but not their major degradation products lacking C-terminal domains, were heavily biotinylated. Therefore, lysine-74 at the C terminus of PsaE was accessible for biotinylation. Similarly, lysine-107, or lysine-118, or both in PsaD could be modified by NHS-biotin.
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PMID:Identification of surface-exposed domains on the reducing side of photosystem I. 799 85

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