Hsp60 is targeted to a cryptic mitochondrion-derived organelle ("crypton") in the microaerophilic protozoan parasite Entamoeba histolytica.
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Entamoeba histolytica is a microaerophilic protozoan parasite in which neither mitochondria nor mitochondrion-derived organelles have been previously observed. Recently, a segment of an E. histolytica gene was identified that encoded a protein similar to the mitochondrial 60-kDa heat shock protein (Hsp60 or chaperonin 60), which refolds nuclear-encoded proteins after passage through organellar membranes. The possible function and localization of the amebic Hsp60 were explored here. Like Hsp60 of mitochondria, amebic Hsp60 RNA and protein were both strongly induced by incubating parasites at 42 degreesC. 5' and 3' rapid amplifications of cDNA ends were used to obtain the entire E. histolytica hsp60 coding region, which predicted a 536-amino-acid Hsp60. The E. histolytica hsp60 gene protected from heat shock Escherichia coli groEL mutants, demonstrating the chaperonin function of the amebic Hsp60. The E. histolytica Hsp60, which lacked characteristic carboxy-terminal Gly-Met repeats, had a 21-amino-acid amino-terminal, organelle-targeting presequence that was cleaved in vivo. This presequence was necessary to target Hsp60 to one (and occasionally two or three) short, cylindrical organelle(s). In contrast, amebic alcohol dehydrogenase 1 and ferredoxin, which are bacteria-like enzymes, were diffusely distributed throughout the cytosol. We suggest that the Hsp60-associated, mitochondrion-derived organelle identified here be named "crypton," as its structure was previously hidden and its function is still cryptic. (+info)
Putidaredoxin-cytochrome p450cam interaction. Spin state of the heme iron modulates putidaredoxin structure.
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During the monooxygenase reaction catalyzed by cytochrome P450cam (P450cam), a ternary complex of P450cam, reduced putidaredoxin, and d-camphor is formed as an obligatory reaction intermediate. When ligands such as CO, NO, and O2 bind to the heme iron of P450cam in the intermediate complex, the EPR spectrum of reduced putidaredoxin with a characteristic signal at 346 millitesla at 77 K changed into a spectrum having a new signal at 348 millitesla. The experiment with O2 was carried out by employing a mutant P450cam with Asp251 --> Asn or Gly where the rate of electron transfer from putidaredoxin to oxyferrous P450cam is considerably reduced. Such a ligand-induced EPR spectral change of putidaredoxin was also shown in situ in Pseudomonas putida. Mutations introduced into the neighborhood of the iron-sulfur cluster of putidaredoxin revealed that a Ser44 --> Gly mutation mimicked the ligand-induced spectral change of putidaredoxin. Arg109 and Arg112, which are in the putative putidaredoxin binding site of P450cam, were essential for the spectral changes of putidaredoxin in the complex. These results indicate that a change in the P450cam active site that is the consequence of an altered spin state is transmitted to putidaredoxin within the ternary complex and produces a conformational change of the 2Fe-2S active center. (+info)
Crystallization and a 5 A X-ray diffraction study of Aphanothece sacrum ferredoxin.
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A chloroplast-type ferredoxin containing two non-heme iron and two labile sulfur atoms per molecule was prepared from Aphanothece sacrum. Crystals were obtained by dialysis against 75% saturated a-monium sulfate solution, and belong to the tetragonal system with cell dimensions a = b = 92.2 A and c = 47.6 A, containing four molecules in an asymmetric unit. The electron density map at 5 A resolution was calculated by using the best phase angles determined by the single isomorphous replacement method coupled with the anomalous dispersion effect. An anomalous dispersion difference Fourier map for the native crystal clearly showed four humps corresponding to the iron atoms in an asymmetric unit. The electron densis surface. (+info)
The superoxide dismutase activity of desulfoferrodoxin from Desulfovibrio desulfuricans ATCC 27774.
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Desulfoferrodoxin (Dfx), a small iron protein containing two mononuclear iron centres (designated centre I and II), was shown to complement superoxide dismutase (SOD) deficient mutants of Escherichia coli [Pianzzola, M.J., Soubes M. & Touati, D. (1996) J. Bacteriol. 178, 6736-6742]. Furthermore, neelaredoxin, a protein from Desulfovibrio gigas containing an iron site similar to centre II of Dfx, was recently shown to have a significant SOD activity [Silva, G., Oliveira, S., Gomes, C.M., Pacheco, I., Liu, M.Y., Xavier, A.V., Teixeira, M., Le Gall, J. & Rodrigues-Pousada, C. (1999) Eur. J. Biochem. 259, 235-243]. Thus, the SOD activity of Dfx isolated from the sulphate-reducing bacterium Desulfovibrio desulfuricans ATCC 27774 was studied. The protein exhibits a SOD activity of 70 U x mg-1, which increases approximately 2.5-fold upon incubation with cyanide. Cyanide binds specifically to Dfx centre II, yielding a low-spin iron species with g-values at 2.27 (g perpendicular) and 1.96 (g parallel). Upon reaction of fully oxidized Dfx with the superoxide generating system xanthine/xanthine oxidase, Dfx centres I and II become partially reduced, suggesting that Dfx operates by a redox cycling mechanism, similar to those proposed for other SODs. Evidence for another SOD in D. desulfuricans is also presented - this enzyme is inhibited by cyanide, and N-terminal sequence data strongly indicates that it is an analogue to Cu,Zn-SODs isolated from other sources. This is the first indication that a Cu-containing protein may be present in a sulphate-reducing bacterium. (+info)
Organization and expression of nitrogen-fixation genes in the aerobic nitrogen-fixing unicellular cyanobacterium Synechococcus sp. strain RF-1.
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Sixteen nif and 'nif-associated' genes (expressed only under conditions of nitrogen fixation) in Synechococcus sp. strain RF-1 have been cloned and sequenced. All of the nif and nif-associated genes identified in Synechococcus RF-1 were arranged in a continuous cluster spanning approximately 18 kb and containing seven operons. The nifH operon (nifH-nifD-nifK) has been reported previously. nifB, fdxN, nifS, nifU and nifP were found to be located upstream of the nifH operon. nifB-fdxN-nifS-nifU were expressed as an operon. A nifP-like gene was found to be located just upstream of nifB. nifE, nifN, nifX, nifW and the nif-associated hesA, hesB and 'fdx' were found to be located downstream from nifK. The genes located downstream from nifK are arranged nifE-nifN-nifX-orf-nifW-hesA-hesB-'+ ++fdx' and span approximately 7 kb. The function of the ORF situated between nifX and nifW is not known. However, it was identified as a counterpart of ORF-2 in Anabaena sp. strain PCC 7120 based on the deduced amino acid sequence. Northern hybridization and primer extension analysis indicated that the nif and nif-associated genes are organized in nifE-nifN, nifX-orf, nifW-hesA-hesB and 'fdx'-containing operons, respectively. According to the results of this study and previous reports, the genes are expressed in a rhythmic pattern with peaks during the dark phase when the culture is grown in a 12 h light/12 h dark regimen. The rhythm persisted after the culture was transferred to continuous illumination. (+info)
A novel aromatic-ring-hydroxylating dioxygenase from the diterpenoid-degrading bacterium Pseudomonas abietaniphila BKME-9.
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Pseudomonas abietaniphila BKME-9 is able to degrade dehydroabietic acid (DhA) via ring hydroxylation by a novel dioxygenase. The ditA1, ditA2, and ditA3 genes, which encode the alpha and beta subunits of the oxygenase and the ferredoxin of the diterpenoid dioxygenase, respectively, were isolated and sequenced. The ferredoxin gene is 9. 2 kb upstream of the oxygenase genes and 872 bp upstream of a putative meta ring cleavage dioxygenase gene, ditC. A Tn5 insertion in the alpha subunit gene, ditA1, resulted in the accumulation by the mutant strain BKME-941 of the pathway intermediate, 7-oxoDhA. Disruption of the ferredoxin gene, ditA3, in wild-type BKME-9 by mutant-allele exchange resulted in a strain (BKME-91) with a phenotype identical to that of the mutant strain BKME-941. Sequence analysis of the putative ferredoxin indicated that it is likely to be a [4Fe-4S]- or [3Fe-4S]-type ferredoxin and not a [2Fe-2S]-type ferredoxin, as found in all previously described ring-hydroxylating dioxygenases. Expression in Escherichia coli of ditA1A2A3, encoding the diterpenoid dioxygenase without its putative reductase component, resulted in a functional enzyme. The diterpenoid dioxygenase attacks 7-oxoDhA, and not DhA, at C-11 and C-12, producing 7-oxo-11, 12-dihydroxy-8,13-abietadien acid, which was identified by 1H nuclear magnetic resonance, UV-visible light, and high-resolution mass spectrometry. The organization of the genes encoding the various components of the diterpenoid dioxygenase, the phylogenetic distinctiveness of both the alpha subunit and the ferredoxin component, and the unusual Fe-S cluster of the ferredoxin all suggest that this enzyme belongs to a new class of aromatic ring-hydroxylating dioxygenases. (+info)
A functional 4-hydroxysalicylate/hydroxyquinol degradative pathway gene cluster is linked to the initial dibenzo-p-dioxin pathway genes in Sphingomonas sp. strain RW1.
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The bacterium Sphingomonas sp. strain RW1 is able to use dibenzo-p-dioxin, dibenzofuran, and several hydroxylated derivatives as sole sources of carbon and energy. We have determined and analyzed the nucleic acid sequence of a 9,997-bp HindIII fragment downstream of cistrons dxnA1A2, which encode the dioxygenase component of the initial dioxygenase system of the corresponding catabolic pathways. This fragment contains 10 colinear open reading frames (ORFs), apparently organized in one compact operon. The enzymatic activities of some proteins encoded by these genes were analyzed in the strain RW1 and, after hyperexpression, in Escherichia coli. The first three ORFs of the locus, designated dxnC, ORF2, and fdx3, specify a protein with a low homology to bacterial siderophore receptors, a polypeptide representing no significant homology to known proteins, and a putative ferredoxin, respectively. dxnD encodes a 69-kDa phenol monooxygenase-like protein with activity for the turnover of 4-hydroxysalicylate, and dxnE codes for a 37-kDa protein whose sequence and activity are similar to those of known maleylacetate reductases. The following gene, dxnF, encodes a 33-kDa intradiol dioxygenase which efficiently cleaves hydroxyquinol, yielding maleylacetate, the ketoform of 3-hydroxy-cis,cis-muconate. The heteromeric protein encoded by dxnGH is a 3-oxoadipate succinyl coenzyme A (succinyl-CoA) transferase, whereas dxnI specifies a protein exhibiting marked homology to acetyl-CoA acetyltransferases (thiolases). The last ORF of the sequenced fragment codes for a putative transposase. DxnD, DxnF, DxnE, DxnGH, and DxnI (the activities of most of them have also been detected in strain RW1) thus form a complete 4-hydroxysalicylate/hydroxyquinol degradative pathway. A route for the mineralization of the growth substrates 3-hydroxydibenzofuran and 2-hydroxydibenzo-p-dioxin in Sphingomonas sp. strain RW1 thus suggests itself. (+info)
Inorganic Fe2+ formation upon Fe-S protein thermodestruction in the membranes of thermophilic cyanobacteria: Mossbauer spectroscopy study.
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A model description of the Mossbauer spectrum (80 K) of native membranes of the thermophilic cyanobacterium Synechococcus elongatus is suggested on the basis of the known values of quadrupole splitting (deltaE(Q)) and isomer shift (deltaFe) for the iron-containing components of the photosynthetic apparatus. Using this approach, we found that heating the membranes at 70-80 K results in a decrease of doublet amplitudes belonging to F(X), F(A), F(B) and ferredoxin and simultaneous formation of a new doublet with deltaE(Q) = 3.10 mm/s and delta-Fe = 1.28 mm/s, typical of inorganic hydrated forms of Fe2+. The inhibition of electron transfer via photosystem I to oxygen, catalyzed by ferredoxin, occurs within the same range of temperatures. The data demonstrate that the processes of thermoinduced Fe2+ formation and distortions in the photosystem I electron transport in the membranes are interrelated and caused mainly by the degradation of ferredoxin. The possible role of Fe2+ formation in the damage of the photosynthetic apparatus resulting from heating and the action of other extreme factors is discussed. (+info)