Expression in Escherichia coli of DNA coding for human tumor necrosis factor

The variants of expression in Escherichia coli of artificial DNA coding for human tumor necrosis factor, an important immune modulator with selective cytotoxic action on a number of transformed cell lines have been described. The DNA was placed under control of either phage M13 promoter of gene for main coat protein or tandem of pair of E. coli tryptophane promoters. It has been shown that E. coli cells harbouring plasmids described with artificial TNF gene provide good level of protein biosynthesis. The protein has been purified by anion exchange chromatography near to homogeneity and used for preparation of monoclonals. As result three hybridomas effectively produced high affinity monoclonal anti-TNF antibodies have been obtained and characterized.     

Folding of dihydrofolate reductase from Escherichia coli

The urea-induced equilibrium unfolding transition of dihydrofolate reductase from Escherichia coli was monitored by UV difference, circular dichroism (CD), and fluorescence spectroscopy. Each of these data sets were well described by a two-state unfolding model involving only native and unfolded forms. The free energy of folding in the absence of urea at pH 7.8, 15 degrees C is 6.13 +/- 0.36 kcal mol-1 by difference UV, 5.32 +/- 0.67 kcal mol-1 by CD, and 5.42 +/- 1.04 kcal mol-1 by fluorescence spectroscopy. The midpoints for the difference UV, CD, and fluorescence transitions are 3.12, 3.08, and 3.18 M urea, respectively. The near-coincidence of the unfolding transitions monitored by these three techniques also supports the assignment of a two-state model for the equilibrium results. Kinetic studies of the unfolding and refolding reactions show that the process is complex and therefore that additional species must be present. Unfolding jumps in the absence of potassium chloride revealed two slow phases which account for all of the amplitude predicted by equilibrium experiments. Unfolding in the presence of 400 mM KCl results in the selective loss of the slower phase, implying that there are two native forms present in equilibrium prior to unfolding. Five reactions were observed in refolding: two slow phases designated tau 1 and tau 2 that correspond to the slow phases in unfolding and three faster reactions designated tau 3, tau 4, and tau 5 that were followed by stopped-flow techniques. The kinetics of the recovery of the native form was monitored by following the binding of methotrexate, a tight-binding inhibitor of dihydrofolate reductase, at 380 nm.(ABSTRACT TRUNCATED AT 250 WORDS)     

The electrostatic potential of Escherichia coli dihydrofolate reductase

Escherichia coli dihydrofolate reductase (DHFR) carries a net charge of -10 electrons yet it binds ligands with net charges of -4 (NADPH) and -2 (folate or dihydrofolate). Evaluation and analysis of the electrostatic potential of the enzyme give insight as to how this is accomplished. The results show that the enzyme is covered by an overall negative potential (as expected) except for the ligand binding sites, which are located inside "pockets" of positive potential that enable the enzyme to bind the negatively charged ligands. The electrostatic potential can be related to the asymmetric distribution of charged residues in the enzyme. The asymmetric charge distribution, along with the dielectric boundary that occurs at the solvent-protein interface, is analogous to the situation occurring in superoxide dismutase. Thus DHFR is another case where the shape of the active site focuses electric fields out into solution. The positive electrostatic potential at the entrance of the ligand binding site in E. coli DHFR is shown to be a direct consequence of the presence of three positively charged residues at positions 32, 52, and 57--residues which have also been shown recently to contribute significantly to electronic polarization of the ligand folate. The latter has been postulated to be involved in the catalytic process. A similar structural motif of three positively charged amino acids that gives rise to a positive potential at the entrance to the active site is also found in DHFR from chicken liver, and is suggested to be a common feature in DHFRs from many species. It is noted that, although the net charges of DHFRs from different species vary from +3 to -10, the enzymes are able to bind the same negatively charged ligands, and perform the same catalytic function.     

Cloning and identification of the Escherichia coli murB DNA sequence, which encodes UDP-N-acetylenolpyruvoylglucosamine reductase.

The murB gene, which complemented the UDP-N-acetylenolpyruvoylglucosamine reductase (EC 1.1.1.158) mutation in Escherichia coli ST5, was cloned from an E. coli chromosomal library. murB was subcloned on a 2.8-kb PvuII fragment into pUC19 and sequenced. A 1,029-bp open reading frame encoded a 342-amino-acid polypeptide of 37,859 Da. A DNA sequence homology search revealed that murB had almost 100% homology with a previously reported unidentified open reading frame, ORFII, at 89.9 min. Physical and genetic mapping results were consistent with this map position, and minicell analyses of murB subclones showed a plasmid-encoded protein of approximately 37,000 Da, which closely matched the calculated size of the murB protein.     

Characterization of the cloned Escherichia coli dihydrofolate reductase

Dihydrofolate reductase (5,6,7,8-tetrahydrofolate: NADP+ oxidoreductase, EC 1.5.1.3) was purified from Escherichia coli strains that carried derivatives of the multicopy recombinant plasmid, pJFM8. The results of enzyme kinetic and two-dimensional gel electrophoresis experiments showed that the cloned enzyme is indistinguishable from the chromosomal enzyme. Therefore it can be concluded that these strains are ideal for use as a source of enzyme for further studies on the biochemistry and regulation of this important enzyme. The plasmid derivatives were constructed by recloning experiments that utilized several restriction endonucleases. From the analysis both of these plasmids and the purified dihydrofolate reductase enzymes it was possible to deduce the location and orientation of the dihydrofolate reductase structural gene on the parent plasmid, pJFM8.     

Cloning of human mitochondrial DNA in Escherichia coli

In order to determine its nucleotide sequence, human mitochondrial DNA (mtDNA) purified from term placentae was cloned in Escherichia coli using the plasmid vector pBR322. The products of an mtDNA MboI digestion (23 fragments ranging in size from 2800 to 25 base-pairs (bp)) were ligated with BamHI-cut pBR322. The ampicillin-resistant tetracycline-sensitive colonies obtained upon transformation of E. coli χ1776 were screened by agarose gel electrophoresis of colony lysates, colony hybridization and restriction analysis. All but MboI fragment 2 were obtained in this way. MboI fragments 5 and 8 were each found only once among the 705 clones screened. All other MboI fragments were approximately equally represented in the population of clones except for a slight bias towards smaller fragments. MboI fragment 2 overlaps with the mtDNA BamHI/EcoRI (1.7 kb³) and the 0.9 kb HinIII fragments. These were cloned in similarly restricted pBR322 to provide a set of clones covering most of the mtDNA molecule. Clones representative of each MboI fragment were shown to be complementary to mtDNA by hybridization to Southern blots of mtDNA digests and were thereby partially mapped. Further mapping was obtained by restriction analysis of mtDNA sequentially degraded by exonuclease III. A collection of recombinant clones has thus been obtained using the mtDNA isolated from a single placenta and is now being used to obtain a complete nucleotide sequence of human mtDNA.     

Purification and properties of Escherichia coli dihydrofolate reductase.

Dihydrofolate reductase has been purified 40-fold to apparent homogeneity from a trimethoprim-resistant strain of Escherichia coli (RT 500) using a procedure that includes methotrexate affinity column chromatography. Determinations of the molecular weight of the enzyme based on its amino acid composition, sedimentation velocity, and sodium dodecyl sulfate gel electrophoresis gave values of 17680, 17470 and 18300, respectively. An aggregated form of the enzyme with a low specific activity can be separated from the monomer by gel filtration; treatment of the aggregate with mercaptoethanol or dithiothreitol results in an increase in enzymic activity and a regeneration of the monomer. Also, multiple molecular forms of the monomer have been detected by polyacrylamide gel electrophoresis. The unresolved enzyme exhibits two pH optima (pH 4.5 and pH 7.0) with dihydrofolate as a substrate. Highest activities are observed in buffers containing large organic cations. In 100 mM imidazolium chloride (pH 7), the specific activity is 47 mumol of dihydrofolate reduced per min per mg at 30 degrees. Folic acid also serves as a substrate with a single pH optimum of pH 4.5. At this pH the Km for folate is 16 muM, and the Vmax is 1/1000 of the rate observed with dihydrofolate as the substrate. Monovalent cations (Na+, K+, Rb+, and Cs+) inhibit dihydrofolate reductase; at a given ionic strength the degree of inhibition is a function of the ionic radius of the cation. Divalent cations are more potent inhibitors; the I50 of BaCl2 is 250 muM, as compared to 125 mM for KCl. Anions neither inhibit nor activate the enzyme.     

Purification of dihydrofolate reductase amplified in Escherichia coli K-12

The Escherichia coli strain carrying pTP 6-10 which was constructed in our previous work (Iwakura, M., et al. (1983) J. Biochem. 93, 927-930) produces more than 400-fold dihydrofolate reductase as compared with the strain without the plasmid. Dihydrofolate reductase was highly purified from the cell-free extract of the plasmid strain simply by two steps; ammonium sulfate fractionation and ion-exchange chromatography. By 10-fold purification, the enzyme was essentially homogeneous as judged by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The restriction map of pTP 6-10 was also determined and the plasmid was shown to have an Ava I, an EcoR I, a Pst I, a Pvu I, and a Pvu II site. Our results indicate that the plasmid strain is suitable as a source of the enzyme and that plasmid pTP 6-10 is promising as a versatile plasmid vector for efficiently yielding the product of the cloned gene.     

The amino-acid sequence of the dihydrofolate reductase of a trimethoprim-resistant strain of Escherichia coli.

The determination of the amino acid sequence of the dihydrofolate reductase from Escherichia coli RT500 is described. The sequence, comprising 159 residues, has been derived from automatic sequencing of the intact protein in conjunction with manual sequencing of lysine-blocked tryptic peptides, Staphylococcus aureus protease peptides, and alpha-lytic protease peptides. Comparison of the sequence with that of the dihydrofolate reductase from a methotrexate-resistant strain of E. coli (MB1428) shows that 145 of the residues are identical. The distribution of the differences along the length of the molecule is discussed.     

Substrate-induced hysteresis in the activity of Escherichia coli dihydrofolate reductase

Full time course studies of the kinetic activity of Escherichia coli dihydrofolate reductase show that there is an increase in activity with time. The half-time for this hysteretic behavior is about 9 s. Preincubation of the enzyme with either of the substrates abolishes the lag and results in initial velocities which are 2-2.3-fold faster than those observed for the non-preincubated enzyme. The kinetic properties of the activated and nonactivated forms of the enzyme appear to be similar as measured by the full time course of the reaction. The results are consistent with observations for NADPH binding studies that the enzyme exists in two interconvertible forms, one of which is incapable of binding NADPH (Cayley, P. J., Dunn, S. M. J., and King, R. W. (1981) Biochemistry 20, 874-879).     

Escherichia coli dihydrofolate reductase: isolation and characterization of two isozymes.

A combination of affinity column chromatography and preparative gel electrophoresis has been used to purify to homogeneity the two isozymes of dihydrofolate reductase from a trimethoprim-resistant strain of Escherichia coli B (RT 500). These enzyme forms are noninterconvertible and are present in crude cell lysates, but other electrophoretic species can be generated durng purification if sulfhydryl-protecting agents, such as dithiothreitol, are not present. The two isozymes, numbered form 1 and form 2 with respect to their decreasing electrophoretic mobilities, have similar molecular weights (18 500), molecular radii (21 A), and apparent Km values for reduced nico inamide adenin- dinucleotide (NADH) and NADH phosphate (NADPH). Both forms contain 2 mol of sulfhydryl/mol of enzyme which can be oxidized to intramolecular disulfide bonds. However, forms 1 and 2 differ physically in their electrophoretic mobility and isoelectric point and kinetically in their pH-activity profile, specific activity, Km for dihydrofolate, and their affinity toward a number of inhibitors.     

DNA sequence of the gene coding for Escherichia coli ribonuclease H

The gene for Escherichia coli ribonuclease H has been studied by use of a plasmid which contains a segment of the E. coli chromosome. The genomic DNA was subcloned from pLC28-22 to pBR322 by use of various restriction enzymes. Such subcloning limited the RNase H gene to a piece of DNA no longer than 760 base pairs. Cells bearing plasmids containing the RNase H gene produce as much as 10-15 times the normal amount of RNase H without any drastic effect on maintenance of the plasmid or cell growth. DNA sequence analysis has permitted the prediction of a protein whose molecular weight is 17,559 (155 amino acid residues). The predicted sequence was confirmed by amino acid analysis, NH2-terminal amino acid sequence, and size determination of highly purified RNase H.     

Expression in Escherichia coli cells of the nucleotide sequence of DNA coding for the bovine lipotropic hormone

A cDNA fragment of bovine proopiomelanocortin coding for beta-lipotropic hormone was joined with a promoter and ribosome binding site of B. amyloliquefaciens and cloned in E. coli in pBR 327 plasmid. The level of beta-lipotropin synthesis in bacterial cells transformed by the obtained plasmid was estimated immunochemically. The level of beta-lipotropin production was shown to be 5 mg per liter of bacterial culture.