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Query: UNIPROT:P06889 (Mol)
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The expression under microaerobic conditions of the Rhizobium meliloti nifA and consequently the nifHDK genes was found to be negatively regulated by ammonia and nitrate. Assimilation of the ammonia to glutamate and glutamine is not required for this regulation to occur. This indicates that ammonia itself, and not a product of its metabolism, may be regulating nif expression. Unlike the situation in Klebsiella pneumoniae, NtrC is apparently not involved in mediating the ammonia effect on nifA expression in R. meliloti. Neither does the fixK gene product, which is known to regulate nifA in R. meliloti, appear to be involved in mediating the ammonia effect. The regulation of nifA by ammonia is shown to be mediated through the FixL protein. A truncated fixJ gene, the product of which has been shown to induce nifA expression irrespective of the oxygen status of the cell, also circumvented the repressive effect of ammonia on nifA expression. This suggests that the ammonia effect is mediated through the FixLJ regulatory cascade. Interestingly no effect of ammonia on fixK expression was observed.
Mol Gen Genet 1992 Sep
PMID:Ammonia regulation of the Rhizobium meliloti nitrogenase structural and regulatory genes under free-living conditions: involvement of the fixL gene product? 140 87

Genetic transformation of cereals by direct DNA delivery via microprojectile bombardment has become an established procedure in recent years. But the derivation of functional transgenic plants, especially in wheat, is still problematic, mainly due to low efficiency of DNA delivery and the reduced regeneration capability of microprojectile-bombarded tissue. We focussed on these two aspects and found that the regeneration of scutellar calli of wheat can be rendered highly efficient and considerably accelerated by a liquid culture phase in screen rafts. We also found that the expression of a reporter gene following DNA delivery by microprojectile can be improved by maintaining the scutellar calli in 0.25 M mannitol before and after bombardment, by bombardment in the presence of silver thiosulfate and Ca(NO3)2 (rather than CaCl2) and by the elimination of spermidine from the DNA/microprojectile mixture. A protocol that includes all these features leads to several-fold higher transient expression of the reporter gene than have previously published procedures.
Mol Gen Genet 1992 Nov
PMID:Improvement of plant regeneration and GUS expression in scutellar wheat calli by optimization of culture conditions and DNA-microprojectile delivery procedures. 146 2

Expression of the narK gene of Escherichia coli, like the narGHJI operon, is positively regulated by two trans-acting factors: Fnr, which is activated by anaerobic conditions, and NarL, which is activated by the conditions, and NarL, which is activated by the presence of nitrate. Unlike the narGHJI operon, the 5' untranslated region of the narK gene contains two putative Fnr-binding-site sequences and two putative NarL-binding-site sequences. To define the role of these putative cis-acting regions, transcription start sites were identified and the effects of promoter region modifications on transcription were determined. Primer extension analysis identified several transcripts for the narK gene expressed from plasmids. Expression from the major promoter, P1, was induced by anaerobic growth conditions and further elevated in the presence of nitrate, while that from a weaker promoter, P2, appeared to be constitutive. The position of the major transcription start site placed one of the putative Fnr-binding sites (Fnr1 box) and one of the NarL-binding sites (NarL2 box) at positions analogous to those previously established for the narGHJI operon promoter region, while the other two binding sites were located in the non-homologous 150 bp sequence which separates the Fnr1 and NarL2 boxes. Based on the effects of selective 5' deletions and site-directed modifications, Fnr-dependent expression was dependent only on the Fnr1 box and nitrate stimulation was dependent on the presence of the NarL2 box. In the absence of the NarL2 box, the NarL1 box did not promote stimulation by nitrate. The Fnr2 box was not required for anaerobic induction of expression but its modification appeared to reduce the level of stimulation by nitrate.
Mol Microbiol 1992 Dec
PMID:Identification of functional cis-acting sequences involved in regulation of narK gene expression in Escherichia coli. 147 1

An open reading frame from Rhizobium leguminosarum bv. viciae strain VF39, previously identified and found to be similar to Escherichia coli fnr and Rhizobium meliloti fixK (orf240, thereafter called fnrN), was further analysed. Analysis of the expression of an fnrN-lacZ transcriptional fusion revealed that fnrN is preferentially expressed under oxygen limitation. Using R. meliloti fixN-lacZ fusions it was shown that the fnrN gene product only mediates transcriptional activation under microaerobiosis, indicating that the FnrN protein responds, directly or indirectly, to oxygen. Plasmids which expressed fnrN under the control of an E. coli promoter were able to complement an E. coli fnr mutant with respect to anaerobic growth on nitrate but not fumarate, and to promote anaerobic but not aerobic activation of the Fnr-dependent E. coli genes narGHJI, nirB and fdnGHI coding for nitrate reductase, NADH-dependent nitrite reductase and formate dehydrogenase-N, respectively. Fumarate and DMSO reductase activities were not induced by FnrN. The E. coli fnr gene substituted for fnrN in oxygen-regulated transcription of nirB- and fixN-lacZ fusions in R. leguminosarum. The results indicate that Fnr and FnrN are functionally very similar and share a common mode of oxygen-dependent transcriptional activation. From hybridization studies, it appeared that fnrN-like genes are present in a number of different R. leguminosarum strains.
Mol Microbiol 1992 Nov
PMID:The Rhizobium leguminosarum FnrN protein is functionally similar to Escherichia coli Fnr and promotes heterologous oxygen-dependent activation of transcription. 148 91

The PRms protein is a pathogenesis-related (PR)-like protein whose mRNA accumulates during germination of maize seeds. Expression of the PRms gene is induced after infection of maize seeds with the fungus Fusarium moniliforme. To further our investigations on the expression of the PRms gene we examined the accumulation of PRms mRNA in different tissues of maize seedlings infected with F. moniliforme and studied the effect of fungal elicitors, the mycotoxin moniliformin, the hormone gibberellic acid, and specific chemical agents. Our results indicate that fungal infection, and treatment either with fungal elicitors or with moniliformin, a mycotoxin produced by F. moniliforme, increase the steady-state level of PRms mRNA. PRms mRNA accumulation is also stimulated by the application of the hormone gibberellic acid or by treatment with silver nitrate, whereas acetylsalicylic acid has no effect. In situ RNA hybridization in isolated germinating embryo sections demonstrates that the PRms gene is expressed in the scutellum, particularly in a group of inner cells, and in the epithelium lying at the interface of the scutellum and the endosperm. The pattern of expression of the PRms gene closely resembles that found for hydrolytic enzymes, being confined to the scutellum and the aleurone layer of the germinating maize seed. Our results suggest that the PRms protein has a function during the normal process of seed germination that has become adapted to serve among the defence mechanisms induced in response to pathogens during maize seed germination.
Mol Gen Genet 1992 Jul
PMID:Expression of the gene encoding the PR-like protein PRms in germinating maize embryos. 149 90

In response to nitrate availability, Escherichia coli regulates the synthesis of a number of enzymes involved in anaerobic respiration and fermentation. When nitrate is present, nitrate reductase (narGHJI) gene expression is induced, while expression of the DMSO/TMAO reductase (dmsABC), fumarate reductase (frdABCD) and fermentation related genes are repressed. The narL and narX gene products are required for this nitrate-dependent control, and apparently function as members of a two-component regulatory system. NarX is a presumed sensor-transmitter for nitrate and possibly molybdenum detection. The presumed response-regulator, NarL, when activated by NarX then binds at the regulatory DNA sites of genes to modulate their expression. In this study a third nitrate regulatory gene, narQ, was identified that also participates in nitrate-dependent gene regulation. Strains defective in either narQ or narX alone exhibited no nitrate-dependent phenotype whereas mutants defective in both narQ and narX were fully inactive for nitrate-dependent repression or activation. In all conditions tested, this regulation required a functional narL gene product. These findings suggest that the narX and narQ products have complementary sensor-transmitter functions for nitrate detection, and can work independently to activate NarL, for eliciting nitrate-dependent regulation of anaerobic electron transport and fermentation functions. The narQ gene was cloned, sequenced, and compared with the narX gene. Both gene products are similar in size, hydrophobicity, and sequence, and contain a highly conserved histidine residue common to sensor-transmitter proteins.
Mol Microbiol 1992 Jul
PMID:Identification and characterization of narQ, a second nitrate sensor for nitrate-dependent gene regulation in Escherichia coli. 150 40

The characterization of mutants that are resistant to the herbicide chlorate has greatly increased our understanding of the structure and function of the genes required for the assimilation of nitrate. Hundreds of chlorate-resistant mutants have been identified in plants, and almost all have been found to be defective in nitrate reduction due to mutations in either nitrate reductase (NR) structural genes or genes required for the synthesis of the NR cofactor molybdenum-pterin (MoCo). The cholorate-resistant mutant of Arabidopsis thaliana, chl2, is also impaired in nitrate reduction, but the defect responsible for this phenotype has yet to be explained. chl2 plants have low levels of NR activity, yet the map position of the chl2 mutation is clearly distinct from that of the two NR structural genes that have been identified in Arabidopsis. In addition, chl2 plants are not thought to be defective in MoCo, as they have near wild-type levels of xanthine dehydrogenase activity, which has been used as a measure of MoCo in other organisms. These results suggest that chl2 may be a NR regulatory mutant. We have examined chl2 plants and have found that they have as much NR (NIA2) mRNA as wild type a variable but often reduced level of NR protein, and one-eighth the NR activity of wild-type plants. It is difficult to explain these results by a simple regulatory model; therefore, we reexamined the MoCo levels in chl2 plants using a sensitive, specific assay for MoCo: complementation of Neurospora MoCo mutant extracts.(ABSTRACT TRUNCATED AT 250 WORDS)
Mol Gen Genet 1992 May
PMID:Identification of two tungstate-sensitive molybdenum cofactor mutants, chl2 and chl7, of Arabidopsis thaliana. 153 67

Two nitrate reductases, NRA and NRZ, are present in Escherichia coli. These isoenzymes have the same alpha beta gamma, subunits composition and have similar size and genetic organization. Corresponding subunits of the complexes share at least 75% identity. By subcloning the different genes and expressing them from separate transcriptional units, we have demonstrated (i) that the translation of the subunits and their assembly are not coupled processes, since subunits produced concomitantly but independently can meet efficiently and associate to form active enzymes, and (ii) that the alpha subunit of a given complex can be replaced by its counterpart from the other isoenzyme to yield an active membrane-bound heterologous enzyme. One such heterologous enzyme, alpha A beta Z gamma Z, has been purified; it is less stable than the native enzymes, more susceptible to thermal denaturation, and shows increased sensitivity to proteolysis. It is also less stably bound to the membrane and, consequently, its activity with physiological electron donors is drastically reduced. The possibility that heterologous nitrate reductases could be formed in vivo is discussed with reference to the existence of porin heterotrimers of the outer membrane proteins OmpC, OmpF and PhoE.
Mol Microbiol 1992 Jan
PMID:Formation of active heterologous nitrate reductases between nitrate reductases A and Z of Escherichia coli. 154 5

Two membrane-bound nitrate reductases, NRA and NRZ, exist in Escherichia coli. Both isoenzymes are composed of three structural subunits, alpha, beta, and gamma encoded by narG/narZ, narH/narY and narI/narV, respectively. The genes are in transcription units which also contain a fourth gene encoding a polypeptide, delta, which is not part of the final enzyme. A strain which is devoid of, or does not express, the nar genes, was used to investigate the role of the delta and gamma polypeptides in the formation and/or processing of the nitrate reductase. When only the alpha and beta polypeptides are produced, an (alpha beta) complex exists which is inactive and soluble. When the alpha, beta and delta polypeptides are produced, the (alpha beta) complex is active with artificial donors such as benzyl viologen but is soluble. When the alpha, beta and gamma polypeptides are produced, the (alpha beta) complex is inactive but partially binds the membrane. It was concluded that the gamma polypeptide is involved in the binding of the (alpha beta) complex to the membrane while the delta polypeptide is indispensable for the (alpha beta) nitrate reductase activity. The activation by the delta polypeptide does not seem to involve the insertion of the redox centres of the enzyme since the purified inactive (alpha beta) complex was shown to contain the four iron-sulphur centres and the molybdenum cofactor, which are normally present in the native purified enzyme. The extreme sensitivity of this inactive complex to thermal denaturation or tryptic treatment favours the idea that the delta polypeptide promotes the correct assembly of the alpha and beta subunits. Although this corresponds to the definition of a chaperone protein this possibility has been rejected. In this study we have also demonstrated that the delta or gamma polypeptide encoded by one nar operon can be substituted successfully for by its respective counterpart from the other nar operon to give an active membrane bound heterologous nitrate reductase enzyme.
Mol Microbiol 1992 Jan
PMID:Involvement of the narJ or narW gene product in the formation of active nitrate reductase in Escherichia coli. 154 6

It is thought that regeneration of the liver provides a state of preparedness for the Shwartzman reaction and contributes to the development of endotoxin-associated massive hepatic necrosis following partial hepatectomy. Therefore we examined endotoxin hepatotoxicity in rats with hepatic regeneration after 35% hepatectomy and in rats with liver cell proliferation induced by lead nitrate. Biochemical and histopathological studies showed no enhanced endotoxin hepatotoxicity in either partially hepatectomized rats or in rats with lead nitrate-induced liver cell proliferation. These results indicate that the development of endotoxin-associated hepatic damage after partial hepatectomy may not relate to regeneration and proliferation of the liver.
Exp Mol Pathol 1992 Feb
PMID:A study of endotoxin-associated hepatotoxicity on proliferating hepatocytes. 154 70


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