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Query: UMLS:C1832526 (PCC)
 5,967 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Membrane vesicles, isolated from osmotic lysates of Azotobacter vinelandii spheroplasts in Tris-acetate buffer, rapidly accumulate calcium in the presence of an oxidizable substrate. The addition of D-lactate to vesicles increases the rate of calcium uptake by 34-fold; L-malate, NADH, NADPH, and reduced phenazine methosulfate are nearly as effective as lactate. The intravesicular calcium pool which accumulates under these conditions is rapidly discharged by isotopic exchange or in the presence of respiratory inhibitors, uncouplers, or EGTA. The uptake rates for calcium follow Michaelis-Menten kinetics yielding a Km of 48 microM and a V max of 45 nmoles/min/mg membrane protein. Initial rates of EGTA-induced calcium efflux also follow saturation kinetics, giving a V max identical to that for calcium entry; but the Km for exodus is 14 mM, assuming that free calcium accumulates in vesicles. The difference in the affinity of calcium for the entry and exit processes observed during respiration is sufficient to account for the estimated 150-fold calcium concentration gradient achieved under steady-state conditions. The uptake system is specific for calcium as opposed to other cations, but zinc and lanthanum are effective competitors. Calcium uptake is blocked when electron is inhibited by exposure of vesicles to p-chlormercuriphenylsulfonate, hydroxyquinoline-N-oxide, or cyanide, or under anoxic conditions. Divalent cation ionophores (A23187 and X537A) and proton ionophores (CCP and gramicidin D) also block calcium transport effectively. The electrogenic potassium ionophore valinomycin has no effect on lactate-dependent calcium uptake in the presence of potassium; but ionophores which induce electroneutral exchange of protons for sodium or potassium (monensin and nigericin, respectively) did block calcium transport in the presence of the appropriate cation. The fluorescence intensity of quinacrine (an amine probe) in the presence of A. vinelandii membrane vesicles is reduced by 25% on addition of lactate; the quenching is blocked by CCP. This indicates that a pH gradient (inside acid) is developed across the vesicle membrane during lactate oxidation. These results indicate that these membrane preparations contain vesicles of inverted topology (with respect to the intact cell) and suggest that calcium transport occurs by means of electroneutral calcium/proton antiport.
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PMID:Respiration-coupled calcium transport by membrane vesicles from Azotobacter vinelandii. 11 11

Heavy-metal-ion- (Cd2+, Cu2+, Pb2+, Hg2+ and Zn2+) or heat (50 degrees C)-stress treatments of the unicellular cyanobacterium Synechococcus sp., strain PCC 6301, under both light and dark conditions led to the accumulation of bis(5'-nucleosidyl)oligophosphates: Ap4A, Ap4G, Ap3A, Ap3G and Ap3Gp2. Under light regimens, the accumulation of Ap4A and Ap4G is more characteristic of heavy-metal-ion-stressed cells, whereas the accumulation of Ap3A, Ap3G and Ap3Gp2 is the dominant feature of heavy-metal-ion or heat-shock treatment during energy deprivation (i.e. in the dark). This accumulation of bisnucleoside oligophosphates supports a model whereby the adenylylated nucleotides are synthesized by the backward reaction of tRNA-aminoacyl synthetases. These nucleotides may also act to switch or modulate cyanobacterial responses under various environmental stress conditions.
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PMID:Alterations in the accumulation of adenylylated nucleotides in heavy-metal-ion-stressed and heat-stressed Synechococcus sp. strain PCC 6301, a cyanobacterium, in light and dark. 190 20

We examined the effect of cadmium, zinc, copper and mercury ions on the rate of total RNA synthesis and on the accumulation of ribosomal RNAs in Synechococcus sp. PCC 6301, an obligate photoautotrophic cyanobacterium. It was found that treatment of cells with growth-inhibitory concentrations of cadmium and zinc ions severely decreased the rate of total RNA synthesis both under light (growing) and dark (non-growing) conditions, while copper and mercury ions had different inhibitory effects under these conditions. Moreover, cadmium and zinc ions substantially inhibited the "normal" processing of high-molecular-mass rRNAs and the in vivo postmaturational cleavage of 23S rRNA in the light and interfered with the accumulation of dark-specific RNAs of 0.33 x 10(6), 0.24 x 10(6) and 0.16 x 10(6) daltons in the dark. On the other hand, copper and mercury ions had a similar effect in the light, but did not abolish the accumulation of dark-specific, rRNA-derived, RNAs in the dark.
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PMID:Heavy metal ion-induced changes of ribosomal RNA synthesis in Synechococcus sp. strain PCC 6301, a cyanobacterium. 248 16

This paper reports the (de novo) construction of mutants of Synechococcus PCC 7942 lacking the repressor (SmtB) of the metallothionein gene, smtA. These smtA+/B- cells are more tolerant to elevated [Zn2+] and [Cd2+] than cells containing an intact metallothionein divergon (smt). Previously selected (by step-wise adaptation) Cd(2+)-tolerant mutants contain additional copies of smtA and possibly other undetected mutations. It is now confirmed that these cells also contain a deletion within 'all' copies of smtB and hence fail to revert to wild type following subculture in medium which has not been supplemented with Cd2+ or Zn2+. Northern analysis showed enhanced accumulation of smtA transcripts, even in the absence of added metal ions in these mutants. An increase in the accumulation of Zn2+ is reported in cells containing an intact metallothionein divergon compared to cells deficient in both smtA and smtB. This supports the assumption that SmtA binds Zn2+ within cyanobacterial cells. We also describe the use of the above mentioned mutants to identify additional factors involved in the regulation of transcription from the smtA operator-promoter.
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PMID:Construction of Zn2+/Cd(2+)-tolerant cyanobacteria with a modified metallothionein divergon: further analysis of the function and regulation of smt. 759 41

In the biosynthetic conversion of glutamate to the tetrapyrrole precursor, delta-aminolevulinic acid (ALA), glutamate is activated at C-1 by glutamyl-tRNA synthetase-catalyzed ligation to tRNAGlu. Glutamyl-tRNA reductase next catalyzes reduction of the activated glutamate to glutamate-1-semialdehyde (GSA), which is then converted to ALA by GSA aminotransferase. Glutamyl-tRNA synthetase is known to require a divalent metal (usually Mg2+) for activity, but it has not been established whether Mg2+ or another metal ion is also required for glutamyl-tRNA reductase or GSA aminotransferase, because these enzymes have previously been assayed in combined incubations containing all factors required for conversion of glutamate to ALA. We now report the metal requirements individually for each of the three enzyme reactions. Glutamyl-tRNA reductase activity in extracts from both Chlorella vulgaris and Synechocystis sp. PCC 6803 was stimulated by Mg2+ and inhibited by EDTA. EDTA-pretreated Chlorella glutamyl-tRNA reductase-containing fraction had very little activity in the absence of added Mg2+, but recovered full activity in incubations containing added Mg2+. The divalent metal requirement could be met by Mg2+, Mn2+, or Ca2+. Maximum activity was reached at approximately 15 mM concentration of each of these metals, and higher concentrations were inhibitory. Zn2+ was inhibitory at micromolar concentrations. Chlorella glutamyl-tRNA synthetase showed a metal requirement that could be met by Mg2+ or Mn2+, but not Ca2+. Maximum activity was reached at approximately 15 mM Mg2+ or Mn2+. Although the presence of 10 mM Ca2+ did not affect the Mg2+ concentration optimum, Ca2+ increased the effectiveness of low concentrations of Mg2+. In contrast to glutamyl-tRNA synthetase and glutamyl-tRNA reductase, Chlorella GSA aminotransferase did not show a metal requirement or inhibition by EDTA. However, EDTA decreased nonenzymatic transformation of GSA to ALA.
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PMID:Metal requirements of the enzymes catalyzing conversion of glutamate to delta-aminolevulinic acid in extracts of Chlorella vulgaris and Synechocystis sp. PCC 6803. 791 10

The growth and toxicity of various Microcystis aeruginosa strains were tested. Six of 14 strains were lethal to mice, five of which produced microcystin. Of these, M. aeruginosa PCC 7806 produced the most toxin per biomass and was thus used to examine the influence of various trace metals on exponential growth rate and production of microcystin. Zinc was shown to be required for optimal growth as well as toxin production. Al, Cd, Cr, Cu, Mn, Ni, and Sn did not significantly affect toxin yield at non-toxic concentrations of the metals. In contrast, iron had a pronounced effect on growth rate and toxin yield. In the absence and at low concentrations of Fe (< or = 2.5 microM), the cells grew much more slowly, but produced 20-40% more toxin. This is in agreement with the hypothesis that production of microcystins may be a response to specific environmental stress conditions.
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PMID:Influence of trace metals on growth and toxin production of Microcystis aeruginosa. 838 15

Eukaryotic metallothioneins (MTs) have been extensively studied, but the precise functions of most of these molecules are not yet fully understood. Prokaryotes are often more tractable for the analysis of gene function and we report here the generation of mutants of Synechococcus PCC 7942 (strain R2-PIM8) deficient in the MT locus, smt. Viability of these mutants, designated R2-PIM8 (smt), reveals that prokaryotic MT performs no "vital" role (such as donation of metals to metallo-proteins) in Synechococcus. R2-PIM8 (smt) has reduced (approximately 5-fold) tolerance to elevated Zn2+, with detectable hypersensitivity to Cd2+. Restoration of Zn2+ tolerance was used as a selectable marker to isolate recombinants derived from R2-PIM8(smt) after reintroduction of a linear DNA fragment containing an uninterrupted smt locus. These smt-complemented cells also exhibited restored Cd2+ tolerance. Hypersensitivity to Cu2+ was not detected in R2-PIM8(smt) indicating independence of Cu2+ resistance from smt mediated metal (Zn2+/Cd2+) tolerance.
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PMID:Construction of Zn2+/Cd2+ hypersensitive cyanobacterial mutants lacking a functional metallothionein locus. 844 Jul 32

A gene (phoV) encoding an alkaline phosphatase from Synechococcus sp. strain PCC 7942 was isolated by screening a plasmid gene bank for expression of alkaline phosphatase activity in Escherichia coli JM103. Two independent clones carrying the same alkaline-phosphatase-encoding gene were isolated. One of these clones (pKW1) was further analysed and the nucleotide sequence of a contiguous 3234 bp DNA fragment was determined. Two complete open reading frames (ORF1 and phoV) and an incomplete ORF3 were identified reading in the same direction. The deduced phoV gene product showed 34% identity to the alkaline phosphatase PhoA from Zymomonas mobilis, and the N-terminal part of the putative ORF3 protein exhibited 57% identity to a protein of unknown function from Frankia sp. Insertional inactivation of the Synechococcus PCC 7942 phoV gene failed, indicating an essential role for either the phoV or the ORF3 gene product. PhoV consists of 550 amino acid residues, resulting in a molecular mass of 61.3 kDa. To overexpress the Synechococcus PCC 7942 phoV gene in E. coli, plasmid pKW1 was transformed into a phoA mutant of E. coli (CC118). In E. coli strain CC118(pKW1) PhoV was expressed constitutively with high rates of activity, and was shown to be membrane associated in the periplasmic space. After partial purification of the recombinant PhoV, it was shown that, like other alkaline phosphatases, the Synechococcus PhoV had a broad pH optimum in the alkaline region and a broad substrate specificity for phosphomonoesters, required Zn2+ for activity, and was inhibited by phosphate. In contrast to several other alkaline phosphatases, PhoV was inhibited by Mn2+. Due to the lack of a Synechococcus PCC 7942 phoV mutant strain, the function of PhoV remains uncertain. However, the present results show that Synechococcus PCC 7942 has a second, probably phosphate-irrepressible, alkaline phosphatase (PhoV, 61.3 kDa) in addition to the phosphate-repressible enzyme (PhoA, 145 kDa) already described.
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PMID:The cyanobacterium Synechococcus sp. strain PCC 7942 contains a second alkaline phosphatase encoded by phoV. 857 98

Plasmids containing DNA from the green photosynthetic bacterium Chlorobium vibrioforme complement a heme-requiring Escherichia coli hemB mutant that is deficient in porphobilinogen (PBG) synthase activity. PBG synthase activity was detected in extract of complemented cells but not in that of cells transformed with control plasmid. The sequence of the C. vibrioforme hemB gene predicts a HemB protein that contains 328 amino acids, has a molecular weight of 36,407, and is 53% identical to the homologous proteins of Synechocystis sp. PCC 6301 and Rhodobacter capsulatus. The response of C. vibrioforme PBG synthase to divalent metals is unlike that of any previously described PBG synthase; Mg2+ stimulates but is not required for activity, and Zn2+ neither stimulates nor is required. This response correlates with predicted sequences of two putative variable metal binding regions of C. vibrioforme HemB. The C. vibrioforme hemB open reading frame begins 1585 bases downstream from the end of the hemD open reading frame and is transcribed in the same direction as hemA, hemC, and hemD. However, hemB is not part of the same transcription unit as these genes, and the hemB transcript is approximately the same size as the hemB gene alone. Between hemD and hemB there is an intervening open reading frame that is oriented in the opposite direction and encodes a protein with a predicted amino acid sequence significantly similar to that of inositol monophosphatase, an enzyme that is not involved in tetrapyrrole biosynthesis. The gene order within hem gene clusters is highly conserved in phylogenetically diverse prokaryotic organisms. This conservation suggests that there are functional constraints on the relative order of the hem genes.
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PMID:Structure and expression of the Chlorobium vibrioforme hemB gene and characterization of its encoded enzyme, porphobilinogen synthase. 862 8

Zn2+ proteins pervade metabolism and are essential for gene expression. However, no proteins have been ascribed the central roles of Zn2+ donation to, or removal from, metalloproteins, or Zn2+ storage in vegetative plant tissue. In animals, such functions have been proposed for metallothioneins. Plants contain multiple metallothionein-like genes but their predicted products, which differ significantly from animal metallothioneins, remain to be isolated from vegetative tissue and their roles are uncertain. The type 2 metallothionein-like gene from Arabidopsis, MT2, was expressed under the control of Zn2+-responsive elements derived from the cyanobacterial metallothionein divergon, smt. Zn2+-dependent expression of MT2 transcripts in Synechococcus PCC 7942 was confirmed by northern analysis. The Arabidopsis MT2 gene partly complemented Zn2+ hypersensitivity in mutants of Synechococcus PCC 7942 which are functionally deficient in an endogenous Zn2+-metallothionein gene, smtA. MT2 was also expressed as a recombinant fusion protein in Escherichia coli, purified and shown to bind Zn2+ in vitro. The mean pH of half displacement of Zn2+ from MT2 was estimated to be 5.05. This suggests that MT2 has a greater affinity for Zn2+ than phytochelatins. The results presented here reveal that MT2 is capable of binding Zn2+ in vitro, conferring tolerance to elevated [Zn2+] in vivo within cyanobacteria and is likely to compete with other polypeptides for cellular Zn2+ in planta.
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PMID:Expression of the type 2 metallothionein-like gene MT2 from Arabidopsis thaliana in Zn(2+)-metallothionein-deficient Synechococcus PCC 7942: putative role for MT2 in Zn2+ metabolism. 870 27


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