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Query: UMLS:C1832526 (PCC)
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The PsaC subunit of Photosystem I (PS I) is a 9.3-kDa protein that binds two important cofactors in photosynthetic electron transfer: the [4Fe-4S] clusters FA and FB. The g-tensor orientation of FA- and FB- is believed to be correlated to the preferential localization of the mixed-valence and equal-valence (ferrous) iron pairs in each [4Fe-4S]+ cluster. The preferential position of the mixed-valence and equal-valence pairs, in turn. can be inferred from the study of the temperature dependence of contact-shifted resonances by 1H NMR spectroscopy. For this, a sequence-specific assignment of these signals is required. The 1H NMR spectrum of reduced, unbound PsaC from Synechococcus sp. PCC 7002 at 280.4 K in 99% D2O solution shows 18 hyperfine-shifted resonances. The non-solvent-exchangeable, hyperfine-shifted resonances of reduced PsaC are clearly identified as belonging to the cysteines coordinating the clusters FA- and FB- by their downfield chemical shifts, by their temperature dependencies, and by their short T1 relaxation times. The usual fast method of assigning the 1H NMR spectra of reduced [4Fe-4S] proteins through magnetization transfer from the oxidized to the reduced state was not feasible in the case of reduced PsaC. Therefore, a de novo self-consistent sequence-specific assignment of the hyperfine-shifted resonances was obtained based on dipolar connectivities from 1D NOE difference spectra and on longitudinal relaxation times using the X-ray structure of Clostridium acidi urici 2[4Fe-4S] cluster ferredoxin at 0.94 A resolution as a model. The results clearly show the same sequence-specific distribution of Curie and anti-Curie cysteines for unbound, reduced PsaC as established for other [4Fe-4S]-containing proteins; therefore, the mixed-valence and equal-valence (ferrous) Fe-Fe pairs in FA- and FB- have the same preferential positions relative to the protein. The analysis reveals that the magnetic properties of the two [4Fe-4S] clusters are essentially indistinguishable in unbound PsaC, in contrast to the PsaC that is bound as a component of the PS I complex.
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PMID:Paramagnetic 1H NMR spectroscopy of the reduced, unbound photosystem I subunit PsaC: sequence-specific assignment of contact-shifted resonances and identification of mixed- and equal-valence Fe-Fe pairs in [4Fe-4S] centers FA- and FB-. 1090 49

Two promoter probe vectors were constructed for the cyanobacterium Synechocystis sp. strain PCC 6803 using reporter genes, which can be easily detected and quantified in vivo by the ability of their encoded proteins to emit light. The vectors allow the transcriptional fusion of promoter sequences with the gfp and luxAB genes, respectively, and their stable integration into a neutral site of the Synechocystis chromosome. Functionality of these vectors was demonstrated by cloning the promoter of the isiAB operon into both promoter probe vectors and analyzing the stress-dependent emission of light by the obtained reporter strains. As was found before for the isiAB operon, the P(isiAB) reporter gene fusions were induced by iron starvation and high salt stress. Induction rates of mRNA of the wild type operon and the reporter gene fusions were found to be essentially the same, indicating that a promoter fragment containing all necessary regulatory elements has been cloned. However, using the gfp gene a slow increase of protein and fluorescence was found, while the luxAB reporter gene constructs led to a rapid increase in luminescence. The same was found after retransfer of cells back into control media, in which the Gfp protein disappeared slowly, while the LuxAB-based luminescence decreased rapidly. These experiments show that both reporter genes can be used in Synechocystis: the luxAB system seems to be favourable regarding reaction time, while the gfp system has the advantage of being independent from any substrate.
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PMID:Construction of promoter probe vectors for Synechocystis sp. PCC 6803 using the light-emitting reporter systems Gfp and LuxAB. 1095 63

A mutant of Synechocystis sp. strain PCC 6803 disrupted for sll1878 exhibited greatly reduced Fe(3+) transport activity. The K(m) value of sll1878-dependent Fe(3+) transport in cells grown in iron-replete medium was 0.5 microM. Both the maximal rate and K(m) value were increased in iron-starved cells.
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PMID:A gene of Synechocystis sp. Strain PCC 6803 encoding a novel iron transporter. 1105 1

Although exposure of Synechococcus sp. PCC 7942 to iron stress induced the accumulation of the isiA gene product (CP43') compared with non-stressed controls, immunodetection of the N-terminus of cytochrome (Cyt) f indicated that iron stress not only reduced the content of the 40 kDa, heme-binding, Cyt f polypeptide by 32% but it also specifically induced the accumulation of a new, 23 kDa, non-heme-binding, putative Cyt f polypeptide. Concomitantly, iron stress restricted intersystem electron transport based on the in vivo reduction of P700(+), monitored as delta A(820)/A(820) in the presence and absence of electron transport inhibitors, as well as the inhibition of the Emerson enhancement effect on O(2) evolution. However, iron stress appeared to be associated with enhanced rates of PS I cyclic electron transport, low rates of PS I-driven photoreduction of NADP(+) but comparable rates for PS II+PS I photoreduction of NADP(+) relative to controls. We hypothesize that Synechococcus sp. PCC 7942 exhibits a dynamic capacity to uncouple PS II and PS I electron transport, which may allow for the higher than expected growth rates observed during iron stress.
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PMID:Iron stress restricts photosynthetic intersystem electron transport in Synechococcus sp. PCC 7942. 1109 62

Pyruvate:ferredoxin (flavodoxin) oxidoreductase (PFO, EC 1.2.7.1) catalyses the oxidative cleavage of pyruvate and coenzyme A to acetylcoenzyme A and CO2 with the simultaneous reduction of ferredoxin or flavodoxin. PFO occurs in anaerobes and in some aerobic archaea and bacteria. For cyanobacteria, activity measurements indicated the occurrence of PFO in heterocystous forms. The completely sequenced genomes of the unicellular Synechocystis sp. PCC 6803 and the heterocystous Anabaena sp. PCC 7120 and Nostoc punctiforme revealed the existence of one PFO (encoded by nifJ) in Synechocystis 6803 and N. punctiforme but two different PFOs, encoded by nifJ1 and nifJ2, in Anabaena. Sequence comparison now indicates that all cyanobacterial PFOs are more closely related to those of anaerobes than to those of aerobes. Reverse transcription-polymerase chain reaction (RT-PCR) experiments show that nifJ is transcribed in the presence of saturating iron concentrations in aerobically grown cells of the unicellular Synechococcus sp. PCC 6301 and Synechocystis 6803. Both nifJ genes are transcribed in aerobically grown Anabaena 7120. These findings are corroborated by luciferase reporter gene analysis of nifJ in Synechococcus sp. PCC 7942. The occurrence of PFO in these cyanobacteria is enigmatic.
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PMID:Molecular evidence for the aerobic expression of nifJ, encoding pyruvate:ferredoxin oxidoreductase, in cyanobacteria. 1116 2

Replacement of the axial histidine ligand with exogenous imidazole has been accomplished in a number of heme protein mutants, where it often serves to complement the functional properties of the protein. In this paper, we describe the effects of pH and buffer ion on the crystal structure of the H175G mutant of cytochrome c peroxidase, in which the histidine tether between the heme and the protein backbone is replaced by bound imidazole. The structures show that imidazole can occupy the proximal H175G cavity under a number of experimental conditions, but that the details of the interaction with the protein and the coordination to the heme are markedly dependent on conditions. Replacement of the tethered histidine ligand with imidazole permits the heme to shift slightly in its pocket, allowing it to adopt either a planar or distally domed conformation. H175G crystallized from both high phosphate and imidazole concentrations exists as a novel, 5-coordinate phosphate bound state, in which the proximal imidazole is dissociated and the distal phosphate is coordinated to the iron. To accommodate this bound phosphate, the side chains of His-52 and Asn-82 alter their positions and a significant conformational change in the surrounding protein backbone occurs. In the absence of phosphate, imidazole binds to the proximal H175G cavity in a pH-dependent fashion. At pH 7, imidazole is directly coordinated to the heme (d(Fe--Im) = 2.0 A) with a nearby distal water (d(Fe--HOH) = 2.4 A). This is similar to the structure of WT CCP except that the iron lies closer in the heme plane, and the hydrogen bond between imidazole and Asp-235 (d(Im--Asp) = 3.1 A) is longer than for WT CCP (d(His--Asp) = 2.9 A). As the pH is dropped to 5, imidazole dissociates from the heme (d(Fe--Im) = 2.9 A), but remains in the proximal cavity where it is strongly hydrogen bonded to Asp-235 (d(Im--Asp) = 2.8 A). In addition, the heme is significantly domed toward the distal pocket where it may coordinate a water molecule. Finally, the structure of H175G/Im, pH 6, at low temperature (100 K) is very similar to that at room temperature, except that the water above the distal heme face is not present. This study concludes that steric restrictions imposed by the covalently tethered histidine restrain the heme and its ligand coordination from distortions that would arise in the absence of the restricted tether. Coupled with the functional and spectroscopic properties described in the following paper in this issue, these structures help to illustrate how the delicate and critical interactions between protein, ligand, and metal modulate the function of heme enzymes.
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PMID:Replacement of the axial histidine ligand with imidazole in cytochrome c peroxidase. 1. Effects on structure. 1117 Apr 52

Synechocystis PCC 6803 contains four genes encoding polypeptides with sequence features of CPx-type ATPases, two of which are now designated pacS and ctaA. We show that CtaA and PacS (but not the related transporters, ZiaA or CoaT) facilitate switching to the use of copper (in plastocyanin) as an alternative to iron (in cytochrome c(6)) for the carriage of electrons within the thylakoid lumen. Disruption of pacS reduced copper tolerance but enhanced silver tolerance, and pacS-mediated restoration of copper tolerance was used to select transformants. Disruption of ctaA caused no change in copper tolerance but reduced the amount of copper cell(-1). In cultures supplemented with 0.2 microm copper, photooxidation of cytochrome c(6) (PetJ) was depressed in wild-type cells but remained elevated in both Synechocystis PCC 6803(ctaA) and Synechocystis PCC 6803(pacS). Conversely, plastocyanin transcripts (petE) were less abundant in both mutants at this [copper]. Synechocystis PCC 6803(ctaA) and Synechocystis PCC 6803(pacS) showed increased iron dependence with impaired growth in deferoxamine mesylate (iron chelator)-containing media. Double mutants also deficient in cytochrome c(6), Synechocystis PCC 6803(petJ,ctaA) and Synechocystis PCC 6803(petJ,pacS), were viable, but the former had increased copper dependence with severely impaired growth in the presence of bathocuproinedisulfonic acid (copper chelator). Analogous transporters are likely to supply copper to plastocyanin in chloroplasts.
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PMID:Two Menkes-type atpases supply copper for photosynthesis in Synechocystis PCC 6803. 1126 84

The expression of the chlorophyll a-binding, iron stress-induced protein IsiA is part of the cyanobacterial response to iron deficiency. A new isiA gene from the filamentous heterocystous cyanobacterial strain, Fischerella muscicola PCC 73103, was identified using standard and inverse PCR. While in unicellular cyanobacterial strains isiA is organized in an operon with isiB (encoding flavodoxin), in Fischerella not an isiB gene but another chlorophyll-binding protein encoding gene was identified downstream of isiA, which shows significant similarities to Pcb-like protein encoding genes known from prochlorophytes. The expression of both genes was clearly activated under iron deficiency. Although isiA and pcbC were independently transcribed, the size of the pcbC transcript indicates a large iron-regulated operon. Beside a 10-fold increase of isiA transcript content iron-starved cells of Fischerella showed a blue-shift in the red chlorophyll a absorption peak. In addition, chlorophyll fluorescence at 77 K was dominated by an emission peak at 685 nm. These features are in accordance with the characteristics of IsiA accumulation in iron-starved unicellular cyanobacteria, suggesting identical IsiA function in heterocystous strains in spite of different genetic organization.
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PMID:The iron-regulated isiA gene of Fischerella muscicola strain PCC 73103 is linked to a likewise regulated gene encoding a Pcb-like chlorophyll-binding protein. 1128 57

Genes encoding polypeptides of an ATP binding cassette (ABC)-type ferric iron transporter that plays a major role in iron acquisition in Synechocystis sp. strain PCC 6803 were identified. These genes are slr1295, slr0513, slr0327, and recently reported sll1878 (Katoh et al., J. Bacteriol. 182:6523-6524, 2000) and were designated futA1, futA2, futB, and futC, respectively, for their involvement in ferric iron uptake. Inactivation of these genes individually or futA1 and futA2 together greatly reduced the activity of ferric iron uptake in cells grown in complete medium or iron-deprived medium. All the fut genes are expressed in cells grown in complete medium, and expression was enhanced by iron starvation. The futA1 and futA2 genes appear to encode periplasmic proteins that play a redundant role in iron binding. The deduced products of futB and futC genes contain nucleotide-binding motifs and belong to the ABC transporter family of inner-membrane-bound and membrane-associated proteins, respectively. These results and sequence similarities among the four genes suggest that the Fut system is related to the Sfu/Fbp family of iron transporters. Inactivation of slr1392, a homologue of feoB in Escherichia coli, greatly reduced the activity of ferrous iron transport. This system is induced by intracellular low iron concentrations that are achieved in cells exposed to iron-free medium or in the fut-less mutants grown in complete medium.
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PMID:Genes essential to iron transport in the cyanobacterium Synechocystis sp. strain PCC 6803. 1129 96

The product of the cyanobacterium Synechocystis sp. PCC 6803 gene slr2097 is a 123 amino acid polypeptide chain belonging to the truncated hemoglobin family. Recombinant, ferric heme-reconstituted Synechocystis sp. PCC 6803 hemoglobin is a low-spin complex whose endogenous hexacoordination gives rise to optical and NMR characteristics reminiscent of cytochrome b(5) [Scott, N. L., and Lecomte, J. T. J. (2000) Protein Sci. 9, 587-597]. In this work, the sequential assignments using (15)N-(13)C-labeled protein, (1)H nuclear Overhauser effects, and longitudinal relaxation data identified His70 as the proximal histidine and His46 as the sixth ligand to the iron ion. It was also found that one of two possible heme orientations within the protein matrix is highly preferred (>90%) and that this orientation is the same as in vertebrate myoglobins. The rate constant for the 180 degrees rotation of the heme within a protein cage to produce the favored isomer was 0.5 h(-1) at 25 degrees C, approximately 35 times faster than in sperm whale myoglobin. Variable temperature studies revealed an activation energy of 132 +/- 4 kJ mol(-1), similar to the value in metaquomyoglobin at the same pH. The rate constant for heme loss from the major isomer was estimated to be 0.01 h(-1) by optical spectroscopy, close to the value for myoglobin and decades slower than in the related Nostoc commune cyanoglobin. The slow heme loss was attributed in part to the additional coordination bond to His46, whereas the relatively fast rate of heme reorientation suggested that this bond was weaker than the proximal His70-Fe bond. The standard reduction potential of the hexacoordinated protein was measured with and without poly-L-lysine as a mediator and found to be approximately -150 mV vs SHE, indicating a stabilization of the ferric state compared to most hemoglobins and b(5) cytochromes.
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PMID:Binding of ferric heme by the recombinant globin from the cyanobacterium Synechocystis sp. PCC 6803. 1137 Dec 18

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