Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.2.1.23 (beta-galactosidase)
 14,648 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The Mu dl (ApR lac) bacteriophage was used to generate mutants of Escherichia coli which were defective in formate hydrogenlyase. Three mutants were chosen for further analysis: they lacked hydrogenase (hydrogen: benzyl viologen oxidoreductase) activity, but produced normal levels of fumarate reductase activity and two- to three-fold reduced levels of benzyl viologen (BV)-dependent formate dehydrogenase activity. Two of them (hydC) were shown to contain about 4-fold reduced amounts of formate hydrogenlyase and fumarate-dependent H2 uptake activities. The third one (hydD) was totally devoid of both activities. Their insertion sites were located at 77 min on the E. coli map. Subdivision of these mutants into two classes was subsequently based on the restoration capacity of hydrogenase activity with high concentration of nickel in the growth media. Addition of 500 microM NiCl2 led to a complete recovery of hydrogenase activity, and to the concomitant restoration of normal BV-linked formate dehydrogenase, formate hydrogenlyase and fumarate-dependent H2 uptake activities in the hydC mutants. The hydD mutant was insensitive to the effect of nickel. Expression of the lac operon in hydC and hydD mutants was induced by anaerobiosis. It was not increased by the addition of formate under anaerobic conditions. The presence of nitrate resulted in slightly reduced beta-galactosidase activities in the hydC mutants, whereas those found in the hydD mutant reached only one third of the level obtained in its absence. Fumarate had no effect on both classes. Moreover, in contrast to the hydD locus, the hydC::Mu dl fusions were found to be dependent upon the positive control exerted by the nirR gene product and were totally repressed by an excess of nickel. In addition, the low levels of overall hydrogenase-dependent activities found in a nirR strain were also relieved by the presence of nickel. Our results strongly suggest that the pleiotropic regulatory gene nirR is essential for the expression of a gene (hydC) involved in either transport or processing of nickel in the cell, whose alteration leads to a loss of hydrogenase activity.
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PMID:Genetic and physiological characterization of new Escherichia coli mutants impaired in hydrogenase activity. 308 8

Fumarate reductase catalyzes the terminal step of anaerobic electron transport with fumarate as a terminal electron acceptor. Transcription of the fumarate reductase (frdABCD) operon in Escherichia coli is repressed in the presence of the preferred terminal electron acceptors, oxygen and nitrate. To identify trans-acting genes involved in regulation by nitrate, a number of E. coli mutants were generated in which expression of a frdA'-'lacZ protein fusion was no longer fully repressed by nitrate. One of these mutants, strain LK23R35, exhibited 17-fold higher beta-galactosidase activity than the wild-type strain when grown anaerobically in the presence of nitrate. When grown aerobically in the presence of nitrate, it contained three- to fourfold more beta-galactosidase activity than the wild-type strain did. Oxygen regulation of frd expression, however, was unaffected by the mutation, since the level of beta-galactosidase activity in both strains was nearly identical when they were grown in the absence of nitrate either aerobically or anaerobically. To confirm that the mutation acts in trans to frdABCD, we measured fumarate reductase levels and found them to parallel FrdA'-beta-galactosidase activity under all growth conditions tested. The effect of the mutation is pleiotropic, since the levels of nitrate reductase in LK23R35 were not induced by the addition of nitrate. The frdR mutant was also derepressed for nitrate control of the trimethylamine-N-oxide reductase and alcohol dehydrogenase enzymes. The mutation maps in a region between trp and hemA at 27 min on the E. coli chromosome. This gene, where we call frdR, is involved in both positive and negative regulation of electron transport and fermentation associated genes. A cloned 4.9-kilobase fragment of chromosomal DNA was found to complement the frdR mutation; both repression of fumarate reductase gene expression and activation of nitrate reductase gene expression were restored.
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PMID:The frdR gene of Escherichia coli globally regulates several operons involved in anaerobic growth in response to nitrate. 327 62

The fumarate reductase enzyme complex, encoded by the frdABCD operon, allows Escherichia coli to utilize fumarate as a terminal electron acceptor for anaerobic oxidative phosphorylation. To analyze the expression of fumarate reductase, protein and operon fusions were constructed between the frdA and the lacZ genes and introduced onto the E. coli chromosome at the lambda attachment site. Expression of beta-galactosidase from either fusion was increased 10-fold during anaerobic versus aerobic cell growth, increased an additional 1.5-fold by the presence of fumarate, the substrate, and decreased 23-fold by nitrate, a preferred electron acceptor. The addition of trimethylamine-N-oxide as an electron acceptor did not significantly alter frdA'-'lacZ expression. Control of frd operon expression is therefore exerted at the transcriptional level in response to the availability of the electron acceptors oxygen, fumarate, and nitrate. Anaerobic induction of frdA'-'lacZ expression was impaired in an fnr mutant and was restored when the fnr+ gene was provided in trans, thus establishing that the fnr gene product, Fnr, is responsible for the anaerobic activation of frd operon expression. Nitrate repression of frdA'-'lacZ expression was observed under either aerobic or anaerobic cell growth conditions in both wild-type and fnr mutant strains, demonstrating that the mechanism for nitrate repression is independent of nitrate respiration and oxygen control imparted by Fnr. Studies performed with a fnr'-'lacZ protein fusion confirmed that the fnr gene is expressed both aerobically and anaerobically. A model is proposed for the regulation of frdABCD operon expression in response to the availability of the alternate terminal electron acceptors oxygen, nitrate, and fumarate.
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PMID:Regulation of Escherichia coli fumarate reductase (frdABCD) operon expression by respiratory electron acceptors and the fnr gene product. 329 18

Hierarchical control ensures that facultative bacteria preferentially use the available respiratory electron acceptor with the most positive standard redox potential. Thus, nitrate is used before other electron acceptors such as fumarate for anaerobic respiration. Nitrate regulation is mediated by the NarX-NarL two-component system, which activates the transcription of operons encoding nitrate respiration enzymes and represses the transcription of operons for other anaerobic respiratory enzymes, including enzymes involved in fumarate respiration. These are fumarate reductase (encoded by the frdABCD operon), fumarase B, which generates fumarate from malate, and the DcuB permease for fumarate, malate, and aspartate. The transcription of the corresponding structural genes is activated by the DcuS-DcuR two-component system in response to fumarate or its dicarboxylate precursors. We report results from preliminary transcription microarray experiments that revealed two previously unknown members of the NarL regulon: the aspA gene encoding aspartate-ammonia lyase, which generates fumarate; and the dcuSR operon encoding the dicarboxylate-responsive regulatory system. We measured beta-galactosidase expression from monocopy aspA-lacZ, frdA-lacZ, and dcuS-lacZ operon fusions in response to added nitrate and fumarate and with respect to the dcuR and narL genotypes. Nitrate, acting through the NarX-NarL regulatory system, repressed the transcription of all three operons. Only frdA-lacZ expression, however, was responsive to added fumarate or a dcuR(+) genotype. Phospho-NarL protein protected operator sites in the aspA and dcuS promoter regions from DNase I cleavage in vitro. The overall results are consistent with the hypothesis that nitrate represses frdA operon transcription not only directly, by repressing frdA promoter activity, but also indirectly, by repressing dcuS promoter activity.
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PMID:Hierarchical control of anaerobic gene expression in Escherichia coli K-12: the nitrate-responsive NarX-NarL regulatory system represses synthesis of the fumarate-responsive DcuS-DcuR regulatory system. 1599 4