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At least 217 records · Page 12Linked to original sources
PubMed1990

Overexpression, purification, and characterization of isochorismate synthase (EntC), the first enzyme involved in the biosynthesis of enterobactin from chorismate.

Isochorismate synthase (EC 5.4.99.6), the entC gene product of Escherichia coli, catalyzes the conversion of chorismate to isochorismate, the first step in the biosynthesis of the powerful iron-chelating agent enterobactin. A sequence-specific deletion method has been used to construct an EntC overproducer, which allows for the purification and characterization of the E. coli isochorismate synthase for the first time. The N-terminal sequence and the subunit molecular weight (43,000) of the polypeptide derived from SDS-polyacrylamide gel electrophoresis agree with those deduced from DNA sequence data. The enzyme is an active monomer with a native molecular weight of 42,000. It was shown that EntC alone is fully capable of catalyzing the interconversion of chorismate and isochorismate in both directions and the associated activity is not affected by EntA of the same biosynthetic pathway as has recently been speculated [Elkins, M. F., & Earhart, C. F. (1988) FEMS Microbiol. Lett. 56, 35; Liu, J., Duncan, K., & Walsh, C.T. (1989) J. Bacteriol. 171, 791; Ozenberger, B. A., Brickman, T.J., & McIntosh, M. A. (1989) J. Bacteriol. 171, 775]. The kinetic constants were determined with Km = 14 microM and kcat = 173 min-1 for chorismate in the forward direction and Km = 5 microM and kcat = 108 min-1 for isochorismate in the backward direction. The equilibrium constant for the reaction derived from the kinetic data is 0.56 with the equilibrium lying toward the side of chorismate, corresponding to a free energy difference of 0.36 kcal/mol between chorismate and isochorismate.(ABSTRACT TRUNCATED AT 250 WORDS)

Base Sequence↗
PubMed1997

Residue-based control of helix shape in beta-peptide oligomers.

Proteins and RNA are unique among known polymers in their ability to adopt compact and well-defined folding patterns. These two biopolymers can perform complex chemical operations such as catalysis and highly selective recognition, and these functions are linked to folding in that the creation of an active site requires proper juxtaposition of reactive groups. So the development of new types of polymeric backbones with well-defined and predictable folding propensities ('foldamers') might lead to molecules with useful functions. The first step in foldamer development is to identify synthetic oligomers with specific secondary structural preferences. Whereas alpha-amino acids can adopt the well-known alpha-helical motif of proteins, it was shown recently that beta-peptides constructed from carefully chosen beta-amino acids can adopt a different, stable helical conformation defined by interwoven 14-membered-ring hydrogen bonds (a 14-helix; Fig. 1a). Here we report that beta-amino acids can also be used to design beta-peptides with a very different secondary structure, a 12-helix (Fig. 1a). This demonstrates that by altering the nature of beta-peptide residues, one can exert rational control over the secondary structure.

Amino Acids↗
PubMed1988

The relationship between inhibition of plasminogen-activator activity and prostatic involution.

The role of plasminogen activators (PAs) as potential mediators of involution of the rat ventral prostate was investigated by using an approach involving the administration in vivo of anti-PA drugs. The prostates of castrated rats, which had been injected daily for 7 days with the anti-PA drugs 6-aminohexanoic acid, tranexamic acid, aprotinin and cortisol, were assayed for PA activity, weight and cell number. In the prostates from the castrated controls, there was a 10-fold increase in the mean PA activity and a 7-fold decrease in cell number relative to that of the non-castrated animals. Although this rise in enzyme activity could be decreased to some extent by all the drugs except aprotinin, only treatment with high doses of tranexamic acid or cortisol had a statistically significant effect. A similar pattern was observed with respect to the relative potency of the drugs in preventing the loss of prostatic weight and cell number after castration. The effects of cortisol were dose-dependent, with complete inhibition of both the rise in PA activity and cell loss occurring at a dose of about 15 mg/day. Since the concentration of the principal intranuclear androgen, dihydrotestosterone, was the same in the prostates from treated and untreated castrated rats, the effects of cortisol are not due to increased retention of this androgen. Rather, the high inverse correlation (r = 0.86) between the cellular concentration of PA activity and the cell population of the prostate implies that PAs are directly associated with prostatic involution and that cortisol, and to a lesser extent tranexamic acid, blocks the involution process through inhibition of PAs.

Aminocaproates↗
PubMed1989

Characterization and use of the potent ribonuclease inhibitor aurintricarboxylic acid for the isolation of RNA from animal tissues.

Inclusion of aurintricarboxylic acid (ATA) in extraction buffers for the isolation of RNA from animal tissues resulted in high yields (0.5-2.0 mg/g of tissue) of undegraded material as judged by agarose-gel-electrophoretic analyses and Northern-blotting experiments. However, ATA bound to nucleic acids, forming stable complexes, and so we have established methods for spectrophotometric quantification of RNA in these coloured complexes, and for easy removal of sufficient ATA to leave RNA in a consistently hybridizable condition at the end of a purification. The use and subsequent removal of ATA was straightforward and gave satisfactory results for all rat tissues tested.

Animals↗
PubMed1990

Nuclear proteins bind conserved elements in the abscisic acid-responsive promoter of a rice rab gene.

We have previously shown that the expression of a rice gene, rab-16A, is responsive to abscisic acid (ABA) and osmotic stress in plant tissues and cultured suspension cells. We demonstrate here that transcriptional elements between -294 and -52 of this gene are sufficient to confer ABA-dependent expression on the chloramphenicol acetyltransferase reporter gene in rice protoplasts. Sequence motifs within this 242-base-pair region of the rab-16A gene are conserved among the 5' upstream regions of other ABA-responsive genes. Gel retardation and DNAse I experiments show nuclear factor(s) binding to these sequences. This correlative data indicate that these motifs are involved in the transcription of the rab genes and suggest that they may be ABA-responsive-elements (ABREs).

Abscisic Acid↗
PubMed1985

Em polypeptide and its messenger RNA levels are modulated by abscisic acid during embryogenesis in wheat.

The effect of abscisic acid (ABA) on the expression of the 'early-methionine-labeled' (Em) polypeptide was examined in cultured, immature wheat (Triticum aestivum, L.) embryos and in developing embryos in planta. A complementary DNA (cDNA) library was constructed from poly(A)-rich RNA from immature embryos cultured in the presence of ABA. ABA-enhanced sequences were first identified by differential colony-blot hybridization, and then verified using RNA slot-blot analysis. Dot-blot hybridization showed that one clone, p1015, was homologous to the previously isolated Em cDNA, pWG432. Electrophoretic analysis of the hybrid-select translation product of p1015 confirmed its identity as an Em sequence. Comparison of the p1015 cDNA insert size and the Em message size, from northern blot analysis, showed that p1015 contained about 87% of the Em sequence. RNA slot-blot analysis and protein electrophoresis showed that Em message, but not Em protein, accumulated at a low, basal level in immature embryos in the absence of ABA. Neither Em message nor Em protein was seen in three-day germinated seedlings. Steady-state levels of Em message and protein increased in immature embryos in the presence of ABA, both in culture and in planta. Regulation appeared to be primarily at the level of transcription or specific message stability. Regulation may also involve specific protein stability, since synthesis of Em protein continued in immature embryos in the absence of ABA, but Em protein did not accumulate in detectable amounts. We conclude that ABA specifically modulates Em message and protein levels in immature embryos, but is probably not responsible for the embryogenic specificity of Em expression.

Abscisic Acid↗
PubMed1990

Stimulation of synaptosomal gamma-aminobutyric acid synthesis by glutamate and glutamine.

gamma-Aminobutyric acid (GABA) synthesis was studied in rat brain synaptosomes by measuring the increase of GABA level in the presence of the GABA-transaminase inhibitor gabaculine. The basal rate of synaptosomal GABA synthesis in glucose-containing medium (25.9 nmol/h/mg of protein) was only 3% of the maximal activity of glutamate decarboxylase (GAD; 804 +/- 83 nmol/h/mg of protein), a result indicating that synaptosomal GAD operates at only a small fraction of its catalytic capacity. Synaptosomal GABA synthesis was stimulated more than threefold by adding 500 microM glutamine. Glutamate also stimulated GABA synthesis, but the effect was smaller (1.5-fold). These results indicate that synaptosomal GAD is not saturated by endogenous levels of its substrate, glutamate, and account for part of the unused catalytic capacity. The greater stimulation of GABA synthesis by glutamine indicates that the GAD-containing compartment is more accessible to extrasynaptosomal glutamine than glutamate. The strong stimulation by glutamine also shows that the rates of uptake of glutamine and its conversion to glutamate can be sufficiently rapid to support GABA synthesis in nerve terminals. Synaptosomes carried out a slow net synthesis of aspartate in glucose-containing medium (7.7 nmol/h/mg of protein). Aspartate synthesis was strongly stimulated by glutamate and glutamine, but in this case the stimulation by glutamate was greater. Thus, the larger part of synaptosomal aspartate synthesis occurs in a different compartment than does GABA synthesis.

4-Aminobutyrate Transaminase↗
PubMed1970

Aurintricarboxylic acid and initiation factors of wheat embryo.

In extracts of wheat embryo, aurintricarboxylic acid inhibits attachment of ribosomes to messenger RNA and has little effect on aminoacyl transfer to peptide. This specificity of inhibition allows (i) the demonstration that the initial phase of amino acid incorporation dependent on RNA of tobacco mosaic virus involves an "initiation" reaction between ribosome and messenger RNA which requires two soluble factors, (ii) an assay facilitating purification of the initiation factors, and (iii) the demonstration that these factors are distinct from aminoacyl transfer enzymes.

Amino Acids↗
PubMed1992

Light and oxygen regulation of the synthesis of bacteriochlorophylls a and c in Chloroflexus aurantiacus.

Control of the synthesis of bacteriochlorophylls (Bchls) a and c by light and oxygen was studied in Chloroflexus aurantiacus grown in batch or chemostat culture with serine as the growth-limiting substrate. For comparison, inhibition by gabaculine of the formation of selected tetrapyrroles was studied. The inhibitory effect of gabaculine decreased in the following order of tetrapyrrole formation: coproporphyrin greater than Bchl c greater than Bchl a. Not only did addition of 5-aminolevulinate (ALA) reverse the inhibition by gabaculine, it also caused an increase in Bchl c content when the cultures grew at high concentrations of ALA. Inhibition of Bchl a, Bchl c, and coproporphyrin formation by oxygen was similar to inhibition by gabaculine. Addition of ALA to aerated cultures led to significant accumulation of coproporphyrin. These results suggest that oxygen inhibits tetrapyrrole formation at a site before ALA formation. Control by light was studied with chemostat cultures transferred from 5 klx to 25 klx. This resulted in only a transient increase of the protein level of the culture, while specific contents of Bchls c and a and the ratio Bchl c/Bchl a decreased to lower steady states. However, the specific content of coproporphyrin increased. Addition of ALA to chemostat cultures adapted to 50 klx increased specific coproporphyrin and Bchl c contents by factors of about 20 and 4, respectively, while the specific Bchl a content was only slightly increased and protein levels were unaffected. Increasing the serine concentration caused an initial increase in the specific Bchl c content, which returned to the original value as soon as the protein content had attained its maximal level. These results suggest that light does not control ALA formation as strictly as oxygen and that competition of biomass formation and tetrapyrrole synthesis for common precursors may be influenced by light.

Aminolevulinic Acid↗
PubMed1985

Aluminon: its limited application as a reagent for the detection of aluminum species.

The triammonium salt of aurin tricarboxylic acid, commonly referred to as aluminon, forms a dye that has been used for the colorimetric determination of Al(III) species. We have reviewed the pertinent literature on the reaction of aluminon with respect to the metallic species that form colored aluminon complexes. The effects of experimental variables, such as time, temperature, and pH, upon the color development of the aluminon complex are also presented. Organic and inorganic species, particularly Be(II) and Fe(III), which can affect color formation, are described. The use of aluminon as a histochemical staining agent for the detection of aluminum requires verification by atomic absorption spectrophotometric analysis or other quantitative techniques.

Aluminum↗
PubMed1979

[Uptake of (14C) leucine and protein synthesis in potato tuber buds and effect of abscisic acid on these processes].

The uptake of [14C] leucine and its incorporation into proteins of dormant and growing potato tuber buds were studied. It was found that the label uptake was increased at the beginning of the growth period, whereas the dynamics of this process were not changed in comparison with the dormant buds samples. The rate of [14C] leucine incorporation into proteins was increased in the growing buds; this increase was not, however, due to the increase in the uptake of the labelled precursor and was probably caused by activation of the protein synthesis. In contrast, the activation of protein synthesis was accompanied by changses is the dynamic incorporation of [14C] leucine into the protein at the end of dormancy. The effect of abscisic acid (10(-7) M) on the protein synthesis was not connected with its action of the uptake of labelled precursor and depended on the physiological state of buds and incubation time. A possible mechanism of regulatory effect of abscisic acid on protein synthesis in potato tuber buds is discussed.

Abscisic Acid↗
PubMed1990

A single cyclohexadienyl dehydrogenase specifies the prephenate dehydrogenase and arogenate dehydrogenase components of the dual pathways to L-tyrosine in Pseudomonas aeruginosa.

Dual biosynthetic pathways diverge from prephenate to L-tyrosine in Pseudomonas aeruginosa, with 4-hydroxyphenylpyruvate and L-arogenate being the unique intermediates of these pathways. Prephenate dehydrogenase and arogenate dehydrogenase activities could not be separated throughout fractionation steps yielding a purification of more than 200-fold, and the ratio of activities was constant throughout purification. Thus, the enzyme is a cyclohexadienyl dehydrogenase. The native enzyme has a molecular weight of 150,000 and is a hexamer made up of identical 25,500 subunits. The enzyme is specific for NAD+ as an electron acceptor, and identical Km values of 0.25 mM were obtained for NAD+, regardless of whether activity was assayed as prephenate dehydrogenase or as arogenate dehydrogenase. Km values of 0.07 mM and 0.17 mM were calculated for prephenate and L-arogenate, respectively. Inhibition by L-tyrosine was noncompetitive with respect to NAD+, but was strictly competitive with respect to either prephenate or L-arogenate. With cyclohexadiene as variable substrate, similar Ki values for L-tyrosine of 0.06 mM (prephenate) and 0.05 mM (L-arogenate) were obtained. With NAD+ as the variable substrate, similar Ki values for L-tyrosine of 0.26 mM (prephenate) and 0.28 mM (L-arogenate), respectively, were calculated. This is the first characterization of a purified, monofunctional cyclohexadienyl dehydrogenase.

Amino Acids, Dicarboxylic↗