Altered kinetic properties of tyrosine-183 to cysteine mutation in glutamine synthetase of anabaena variabilis strain SA1 is responsible for excretion of ammonium ion produced by nitrogenase

A L-methionine- D, L-sulfoximine-resistant mutant of the cyanobacterium Anabaena variabilis, strain SA1, excreted the ammonium ion generated from N(2) reduction. In order to determine the biochemical basis for the NH(4)(+)-excretion phenotype, glutamine synthetase (GS) was purified from both the parent strain SA0 and from the mutant. GS from strain SA0 (SA0-GS) had a pH optimum of 7.5, while the pH optimum for GS from strain SA1 (SA1-GS) was 6.8. SA1-GS required Mn(+2) for optimum activity, while SA0-GS was Mg(+2) dependent. SA0-GS had the following apparent K(m) values at pH 7.5: glutamate, 1.7 m M; NH(4)(+), 0.015 m M; ATP, 0.13 m M. The apparent K(m) for substrates was significantly higher for SA1-GS at its optimum pH (glutamate, 9.2 m M; NH(4)(+), 12.4 m M; ATP, 0.17 m M). The amino acids alanine, aspartate, cystine, glycine, and serine inhibited SA1-GS less severely than the SA0-GS. The nucleotide sequences of glnA (encoding glutamine synthetase) from strains SA0 and SA1 were identical except for a single nucleotide substitution that resulted in a Y183C mutation in SA1-GS. The kinetic properties of SA1-GS isolated from E. coli or Klebsiella oxytoca glnA mutants carrying the A. variabilis SA1 glnA gene were also similar to SA1-GS isolated from A. variabilis strain SA1. These results show that the NH(4)(+)-excretion phenotype of A. variabilis strain SA1 is a direct consequence of structural changes in SA1-GS induced by the Y183C mutation, which elevated the K(m) values for NH(4)(+) and glutamate, and thus limited the assimilation of NH(4)(+) generated by N(2) reduction. These properties and the altered divalent cation-mediated stability of A. variabilis SA1-GS demonstrate the importance of Y183 for NH(4)(+) binding and metal ion coordination.     

Production of optically pure D-lactic acid in mineral salts medium by metabolically engineered Escherichia coli W3110

The resistance of polylactide to biodegradation and the physical properties of this polymer can be controlled by adjusting the ratio of L-lactic acid to D-lactic acid. Although the largest demand is for the L enantiomer, substantial amounts of both enantiomers are required for bioplastics. We constructed derivatives of Escherichia coli W3110 (prototrophic) as new biocatalysts for the production of D-lactic acid. These strains (SZ40, SZ58, and SZ63) require only mineral salts as nutrients and lack all plasmids and antibiotic resistance genes used during construction. D-Lactic acid production by these new strains approached the theoretical maximum yield of two molecules per glucose molecule. The chemical purity of this D-lactic acid was approximately 98% with respect to soluble organic compounds. The optical purity exceeded 99%. Competing pathways were eliminated by chromosomal inactivation of genes encoding fumarate reductase (frdABCD), alcohol/aldehyde dehydrogenase (adhE), and pyruvate formate lyase (pflB). The cell yield and lactate productivity were increased by a further mutation in the acetate kinase gene (ackA). Similar improvements could be achieved by addition of 10 mM acetate or by an initial period of aeration. All three approaches reduced the time required to complete the fermentation of 5% glucose. The use of mineral salts medium, the lack of antibiotic resistance genes or plasmids, the high yield of D-lactate, and the high product purity should reduce costs associated with nutrients, purification, containment, biological oxygen demand, and waste treatment.     

Biophysical characterization of the interactions of HTI-286 with tubulin heterodimer and microtubules

HTI-286 is a synthetic analogue of the natural product hemiasterlin and is a potent antimitotic agent. HTI-286 inhibits the proliferation of tumor cells during mitosis. The observed antimitotic activity is due to the binding of HTI-286 to tubulin. This report details the effects of HTI-286 on soluble tubulin and preassembled microtubules. HTI-286 binds tubulin monomer and oligomerizes it to an 18.5 S species corresponding to a discrete ring structure consisting of about 13 tubulin units as determined by sedimentation equilibrium analyses. The rate of formation of the oligomers is dependent on the concentration of HTI-286 and the time of incubation. Tubulin oligomers, specifically the 18.5 S species, form slowly. The interactions of HTI-286 with tubulin were studied by isothermal titration calorimetry. HTI-286 binds tubulin rapidly, and the initial association of HTI-286 with tubulin is enthalpically driven with a DeltaH value of -14 kcal/mol at 25 degrees C and a dissociation constant of ca. 100 nM. However, the accompanying tubulin oligomerization event does not produce measurable heats at 25 degrees C. The dissociation constant estimated from the changes in the intrinsic fluorescence of tubulin was found to be consistent with the calorimetric results. Both HTI-286 and hemiasterlin bind tubulin with nearly equal potency. However, the stability of the tubulin oligomers is not identical under size-exclusion column chromatographic conditions. The tubulin oligomers formed in the presence of HTI-286 dissociate on the column, while the corresponding oligomers formed in the presence of hemiasterlin are stable. Tubulin undergoes a change in the secondary structure in the presence of HTI-286, which is evidenced by changes in the circular dichroic absorption spectrum of tubulin. In contrast to the microtubule-stabilizing effects of paclitaxel, both HTI-286 and hemiasterlin depolymerize preassembled microtubules at micromolar concentrations.     

Decreasing the level of ethyl acetate in ethanolic fermentation broths of Escherichia coli KO11 by expression of Pseudomonas putida estZ esterase

During the fermentation of sugars to ethanol relatively high levels of an undesirable coproduct, ethyl acetate, are also produced. With ethanologenic Escherichia coli strain KO11 as the biocatalyst, the level of ethyl acetate in beer containing 4.8% ethanol was 192 mg liter(-1). Although the E. coli genome encodes several proteins with esterase activity, neither wild-type strains nor KO11 contained significant ethyl acetate esterase activity. A simple method was developed to rapidly screen bacterial colonies for the presence of esterases which hydrolyze ethyl acetate based on pH change. This method allowed identification of Pseudomonas putida NRRL B-18435 as a source of this activity and the cloning of a new esterase gene, estZ. Recombinant EstZ esterase was purified to near homogeneity and characterized. It belongs to family IV of lipolytic enzymes and contains the conserved catalytic triad of serine, aspartic acid, and histidine. As expected, this serine esterase was inhibited by phenylmethylsulfonyl fluoride and the histidine reagent diethylpyrocarbonate. The native and subunit molecular weights of the recombinant protein were 36,000, indicating that the enzyme exists as a monomer. By using alpha-naphthyl acetate as a model substrate, optimal activity was observed at pH 7.5 and 40 degrees C. The Km and Vmax for alpha-naphthyl acetate were 18 microM and 48.1 micromol. min(-1). mg of protein(-1), respectively. Among the aliphatic esters tested, the highest activity was obtained with propyl acetate (96 micromol. min(-1). mg of protein(-1)), followed by ethyl acetate (66 micromol. min(-1). mg of protein(-1)). Expression of estZ in E. coli KO11 reduced the concentration of ethyl acetate in fermentation broth (4.8% ethanol) to less than 20 mg liter(-1).     

Physiological and Genetic Analyses Leading to Identification of a Biochemical Role for the moeA (Molybdate Metabolism) Gene Product in Escherichia coli

A unique class of chlorate-resistant mutants of Escherichia coli which produced formate hydrogenlyase and nitrate reductase activities only when grown in medium with limiting amounts of sulfur compounds was isolated. These mutants failed to produce the two molybdoenzyme activities when cultured in rich medium or glucose-minimal medium. The mutations in these mutants were localized in the moeA gene. Mutant strains with polar mutations in moeA which are also moeB did not produce active molybdoenzymes in any of the media tested. moeA mutants with a second mutation in either cysDNCJI or cysH gene lost the ability to produce active molybdoenzyme even when grown in medium limiting in sulfur compounds. The CysDNCJIH proteins along with CysG catalyze the conversion of sulfate to sulfide. Addition of sulfide to the growth medium of moeA cys double mutants suppressed the MoeA⁻ phenotype. These results suggest that in the absence of MoeA protein, the sulfide produced by the sulfate activation/reduction pathway combines with molybdate in the production of activated molybdenum. Since hydrogen sulfide is known to interact with molybdate in the production of thiomolybdate, it is possible that the MoeA-catalyzed activated molybdenum is a form of thiomolybdenum species which is used in the synthesis of molybdenum cofactor from Mo-free molybdopterin.Molybdoenzymes play essential metabolic roles in most organisms from bacteria to plants and animals (34). All molybdoenzymes other than dinitrogenase contain molybdenum cofactor, which consists of a unique molybdopterin (MPT) complexed with molybdenum (1, 12, 23, 31, 34). In Escherichia coli, the biologically active form of the cofactor in molybdoenzymes is MPT guanine dinucleotide (MGD) (5, 22, 23). Synthesis of this cofactor in an active form requires transport of molybdate into the cell, activation of molybdate, synthesis of the MPT moiety, and incorporation of molybdate into MPT. Although molybdate transport and the various steps in the organic part of MGD biosynthesis are well characterized (17, 24, 33; see references 10, 22, and 23 for reviews), very little is known about the activation and incorporation of molybdenum into the cofactor (22).Mutants which are defective in molybdate metabolism can be isolated as chlorate-resistant mutants (8, 9). A large fraction of these mutants are pleiotropic for all molybdoenzyme activities in the cell, and these comprise the three genetic loci involved in MGD synthesis, moa, mob, and moeB (see references 10, 22, 29, and 31 for reviews). The mod gene products comprise the molybdate transport system through which molybdate is transported into the cell and the Mod⁻ phenotype can be suppressed by increasing molybdate concentration in the medium. The mog mutants which produced formate hydrogenlyase (FHL) activity containing the molybdoenzyme formate dehydrogenase-H (FDH-H) but not nitrate reductase activity was proposed to be defective in molybdochelatase (13, 32). This molybdochelatase is apparently required for production of active nitrate reductase and not for FDH-H.The moe operon codes for two proteins, and only the physiological role of the second gene product, MoeB protein, is known. The MoeB protein activates MPT synthase, which catalyzes the conversion of MPT precursor (precursor Z) to MPT by introducing the needed sulfur to which Mo is coordinated in the molybdenum cofactor (20, 22). The MoeB protein, MPT synthase sulfurylase, is the known S donor in the activation of MPT synthase. The physiological role of MoeA protein coded by the first gene in the two member moe operon is not known. Mutants which are defective in moeA (chlE [29]) produced about 6% of the wild-type levels of MPT (12), although no molybdoenzyme activity was found in these moeA mutants. Since the MoeB protein acts as an S donor in MPT synthesis, it is possible that the first gene product, MoeA protein, also has a similar role in linking S metabolism and Mo metabolism in the cell.During our analysis of molybdate transport-defective mutants, we identified a subgroup of chlorate-resistant mutants with a unique phenotype. Mutations in this class of mutants were mapped in the moeA gene at 18.6 min on the E. coli chromosome (3, 18). The MoeA⁻ phenotype was suppressed when the growth medium was supplemented with sulfide. In this report, we present the physiological and genetic characteristics of E. coli moeA mutants and propose a role for the MoeA protein in the activation of molybdenum by sulfurylation.(This work was presented at the International Symposium on Nitrogen Assimilation: Molecular and Genetic Aspects, 3 to 9 May 1997, Tampa, Fla.)     

Production of nitrous oxide from nitrite in Klebsiella pneumoniae: mutants altered in nitrogen metabolism.

Under anaerobic conditions, Klebsiella pneumoniae reduced nitrite (NO2-), yielding nitrous oxide (N2O) and ammonium ions (NH4+) as products. Nitrous oxide formation accounted for about 5% of the total NO2- reduced, and NH4+ production accounted for the remainder. Glucose and pyruvate were the electron donors for NO2- reduction to N2O by whole cells, whereas glucose, NADH, and NADPH were found to be the electron donors when cell extracts were used. On the one hand, formate failed to serve as an electron donor for NO2- reduction to N2O and NH4+, whereas on the other hand, formate was the best electron donor for nitrate reduction in either whole cells or cell extracts. Mutants that are defective in the reduction of NO2- to NH4+ were isolated, and these strains were found to produce N2O at rates comparable to that of the parent strain. These results suggest that the nitrite reductase producing N2O is distinct from that producing NH4+. Nitrous oxide production from nitric oxide (NO) occurred in all mutants tested, at rates comparable to that of the parent strain. This result suggests that NO reduction to N2O, which also uses NADH as the electron donor, is independent of the protein(s) catalyzing the reduction of NO2- to N2O.     

Amino acids as repressors of nitrogenase biosynthesis in Klebsiella pneumoniae

Nitrogenase biosynthesis in Klebsiella pneumoniae including mutant strains, which produce nitrogenase in the presence of NH₄⁺ (Shanmugam, K.T., Chan, Irene, and Morandi, C. (1975) Biochim. Biophys. Acta 408, 101–111) is repressed by a mixture of L-amino acids. Biochemical analysis shows that glutamine synthetase activity in strains SK-24, SK-28, and SK-29 is also repressed by amino acids, with no detectable effect on glutamate dehydrogenase. Among the various amino acids, L-glutamine in combination with L-aspartate was found to repress nitrogenase biosynthesis completely. In the presence of high concentrations of glutamine (1 mg/ml) even NH₄⁺ repressed nitrogenase biosynthesis in the strains SK-27, SK-37, SK-55 and SK-56. Under these conditions, increased glutamate dehydrogenase activity was also detected. Physiological studies show that nitrogenase derepressed strains are unable to utilize NH₄⁺ as sole source of nitrogen for biosynthesis of glutamate, whereas back mutations leading to NH₄⁺ utilization results in sensitivity to repression by NH₄⁺. These findings suggest that amino acids play an important role as regulators of nitrogen fixation.     

Synthesis of the adenovirus-coded DNA binding protein in infected cells.

Synthesis of the 75K (75K indicates a moleculatr weight of 70,000 to 75,000) DNA binding protein, an early virus-coded protein in adenovirus 2-infected KB cells, and its regulation were studied by using a radioimmune precipitation inhibition assay. The protein was first detected at 4 h postinfection and accumulated at an expoential rate. An arrest of further synthesis (accumulation) was observed at 10 to 11 h postinfection, coinciding with the onset of synthesis of late virion proteins. In contrast, when the infected cells were treated with 25 mug of arabinosyl cytosine per ml to block viral DNA replication, the synthesis of 75K protein did not cease but continue for up to 36 h postinfection. The synthesis of 75K protein in cells after release from a cycloheximide block (2 to 9 h postinfection) was analyzed. Increased amounts of early adenovirus-specific mRNA accumulate in infected cells during a cycloheximide block (Parsons and Green, 1971). However, cycloheximide treatment did not produce increased levels of 75K protein, and an abrupt arrest of 75K protein formation was again observed at the time of synthesis of late virion proteins. Partition of the 75K protein between the nuclear and cytoplasmic fractions during the course of infection was studied. The 75K protein appeared first in the cytoplasm and then in the nucleus after a slight lag. Accumulation of the 75K protein continued both in the cytoplasm and nucleus, with higher levels being found in the cytoplasm.