A dihydrofolate reductase gene from Candida albicans: molecular cloning

The dihydrofolate reductase gene from Candida albicans has been cloned and partially characterized. A genomic bank from C. albicans strain 10127/5 was constructed in Escherichia coli and screened for trimethoprim resistance. A plasmid pMF1, carrying the resistance marker was isolated and characterized by restriction mapping and Southern blotting. Cells harbouring pMF1 were as sensitive as the parental cells to a wide spectrum of antibacterial agents, except for trimethoprim; the dihydrofolate reductase activity from these cells was trimethoprim resistant.     

Molecular cloning of the gene for dihydrofolate reductase from Lactobacillus casei

The Lactobacillus casei gene for dihydrofolate reductase has been cloned in Escherichia coli using the multicopy vector pBR322. A restriction map of the cloned DNA has been prepared. The cloned DNA directs the synthesis of L. casei dihydrofolate reductase in E. coli and confers trimethoprim and methotrexate resistance.     

Isolation of a small DNA fragment carrying the gene for a dihydrofolate reductase from a trimethoprim resistance factor

Summary DNA fragments of the R factor R388 which renders E. coli resistant to trimethoprim by inducing a trimethoprim resistant dihydrofolate reductase (Amyes and Smith, 1974) were inserted into plasmids and screened for the expression of the trimethoprim resistance gene. By means of a two step deletion procedure a 1770 bp EcoRI/BamH1 fragment was isolated which conferred drug resistance and which was found to induce the synthesis of the same dihydrofolate reductase as the parental R factor. Gene dosage experiments indicated that the induction was due to the presence of a dihydrofolate reductase structural gene on the 1770 bp fragment. The gene could be assigned to a segment which was less than 1200 bp long. The 1770 bp fragment and a recombinant plasmid consisting of pSF2124 and part of R388 were mapped with several restriction nucleases. The R factor induced enzyme was partially purified from a strain carrying a multicopy recombinant plasmid into which the 1770 bp fragment was inserted and which induced high levels of dihydrofolate reductase. The enzyme was found to be stable at 100°. Some aspects of the synthesis of dihydrofolate reductase are discussed.Dedicated to Professor Peter Karlson on the occasion of his 60th birthday     

Two distinct types of trimethoprim-resistant dihydrofolate reductase specified by R-plasmids of different compatibility groups.

R-Plasmids from a number of trimethoprim-resistant Escherichia coli and Citrobacter sp. were studied after transfer to E. coli K12 hosts. Each was found to specify a dihydrofolate reductase which was resistant to trimethoprim and Methotrexate, and which could be completely separated from the host chromosomal enzyme by gel filtration. Two distinct types of R-plasmid dihydrofolate reductases were identified. Type I enzymes, typified by the R483 enzyme previously described (Sk?ld, O., and Widh, A. (1974) J. Biol. Chem. 249, 4324-4325), are synthesized in amounts severalfold higher than the chromosomal enzyme. The 50% inhibitory concentrations (I50) of trimethoprim, Methotrexate, and aminopterin are increased several thousandfold over the corresponding values for the chromosomal enzyme. Type II R-plasmid dihydrofolate reductases are synthesized in about the same amount, or less, as the chromosomal enzyme, but are practically several hundredfold higher than those for the type I enzymes. Both types of R-plasmid dihydrofolate reductase showed little difference from the chromosomal enzyme in the binding of dihydrofolate, NADPH, folic acid, and 2,4-diaminopyrimidine.     

Characterization of an R-plasmid dihydrofolate reductase with a monomeric structure

A plasmid-encoded dihydrofolate reductase that originated in a clinical isolate of Salmonella typhimurium (phage type 179) moderately resistant to trimethoprim has been isolated and characterized. The dihydrofolate reductase (called type III) was purified to homogeneity using a combination of gel filtration, hydrophobic chromatography, and methotrexate affinity chromatography. Polyacrylamide gel electrophoresis under denaturing and nondenaturing conditions indicated that the enzyme is a 16,900 molecular weight monomeric protein. Kinetic analyses showed that trimethoprim is a relatively tight binding inhibitor (Ki = 19 nM) competitive with dihydrofolate. The enzyme is also extremely sensitive to methotrexate inhibition (Ki = 9 pM) and has a high affinity for dihydrofolate (Km = 0.4 microM). The sequence of the first 20 NH2-terminal residues of the protein shows 50% homology with the trimethoprim-sensitive chromosomal Escherichia coli dihydrofolate reductase and suggests that the two enzymes may be closely related. This is the first example of a plasmid encoding for a monomeric dihydrofolate reductase only moderately resistant to trimethoprim, and a resistance mechanism, dependent in part on the high dihydrofolate affinity of the type III enzyme, is proposed.     

A new mechanism of plasmid trimethoprim resistance. Characterization of an inducible dihydrofolate reductase

The dihydrofolate reductase encoded by plasmid pUK1123, which confers only a moderate level of trimethoprim resistance on its host, has been isolated and characterized. This enzyme, designated type IV, differs markedly from all previously described plasmid dihydrofolate reductases. It has a relatively high molecular weight of 46,700 as measured by gel filtration and, unlike previous plasmid dihydrofolate reductases, its synthesis is induced in the presence of increasing concentrations of trimethoprim. It is only slightly resistant to trimethoprim but is competitively inhibited by this drug with an inhibitor binding constant of 63 nM. In addition, the enzyme has a relatively low affinity for the substrate, dihydrofolate (Km = 37 microM). This is the first report of a plasmid trimethoprim resistance mechanism resulting from the induced synthesis of a large molecular weight dihydrofolate reductase which is only slightly resistant to trimethoprim. The possible origins of the type IV enzyme are discussed.     

Low level of cross-resistance between triclosan and antibiotics in Escherichia coli K-12 and E. coli O55 compared to E. coli O157

Misuse of biocides has encouraged the emergence of resistance and cross-resistance in certain strains. This study investigated resistance of triclosan-adapted Escherichia coli K-12 and E. coli O55 to antimicrobial agents and compared these to E. coli O157:H7. Cross-resistance in E. coli K-12 and E. coli O55 was observed however to a lesser extent than in E. coli O157:H7. Triclosan-adapted E. coli K-12 demonstrated cross-resistance to chloramphenicol, whereas triclosan-adapted E. coli O55 exhibited resistance to trimethoprim. In comparison, E. coli O157:H7 was resistant to chloramphenicol, tetracycline, amoxicillin, amoxicillin/clavulanic acid, trimethoprim, benzalkonium chloride and chlorohexidine suggesting strain specific rather than general resistance mechanisms.     

Cloning and mapping of the dihydrofolate reductase gene of Bacillus subtilis

The structural gene for dihydrofolate reductase (dfrA) from the Bacillus subtilis 168 chromosome has been cloned, along with the thyB gene, on a 4.5-kb insert contained on chimeric plasmid pER1. The presence of the dfrA gene on pER1 was demonstrated by showing that: (i) transformation of Escherichia coli strains RUE10(Thy-) and RUE11(Thy+) with pER1 resulted in a 60 to 130-fold increase in dihydrofolate reductase (DFRase) activity with a turnover number characteristic of that of B. subtilis and (ii) pER1-mediated transformation of trimethoprim-resistant E. coli strain D05, which overproduced a DFRase with a decreased affinity for trimethoprim, resulted in a 41-fold increase in DFRase activity with an affinity for trimethoprim similar to that of the B. subtilis enzyme. The dfrA gene was mapped to the 200 degrees region of the B. subtilis chromosome, and the gene order was established as thyB dfrA ilvA. Furthermore, the dfrA gene was shown to be linked closely (95-99% cotransformation) to the thyB gene.     

Molecular cloning of a chromosomal DNA region encompassing the dihydrofolate reductase gene ofStreptococcus pneumoniae

Natural competence ofStreptococcus pneumoniae was used to locate and enrich DNA restriction fragments, biologically active for transformation of thymidine-deficient to thymidine-proficient cells. Mutations in the dihydrofolate reductase gene are accompanied by resistance to the drug trimethoprim (Tp). A 6.5-kb region of the pneumococcal chromosome encompassing the dihydrofolate reductase gene has been cloned in plasmid pLS1.Escherichia coli mutants, resistant to Tp, became fully sensitive to the drug when they harbored the recombinant plasmid. The pneumococcaldfrA mutation has been mapped within a 500-bp DNA region.     

Molecular cloning and characterization of the recA gene of Pseudomonas aeruginosa PAO.

The recA gene of Pseudomonas aeruginosa PAO has been isolated and introduced into Escherichia coli K-12. Resistance to killing by UV irradiation was restored in several RecA-E. coli K-12 hosts by the P. aeruginosa gene, as was resistance to methyl methanesulfonate. Recombination proficiency was also restored, as measured by HfrH-mediated conjugation and by the ability to propagate Fec-phage lambda derivatives. The cloned P. aeruginosa recA gene restored both spontaneous and mitomycin C-stimulated induction of lambda prophage in lysogens of a recA strain of E. coli K-12.     

Stereospecific production of the herbicide phosphinothricin (glufosinate) by transamination: cloning, characterization, and overexpression of the gene encoding a phosphinothricin-specific transaminase from Escherichia coli.

We have cloned the gene encoding a 43-kilodalton transaminase from Escherichia coli K-12 with a specificity for L-phosphinothricin [L-homoalanine-4-yl-(methyl)phosphinic acid], the active ingredient of the herbicide Basta (Hoechst AG). The structural gene was isolated, together with its own promoter, and shown to be localized on a 1.6-kilobase DraI-BamHI fragment. The gene is subject to catabolite repression by glucose; however, repression could be relieved completely when 4-aminobutyrate (GABA) served as the sole nitrogen source. The regulation pattern obtained and a comparison of the restriction map of the initially cloned 15-kilobase SalI fragment with the physical map of the E. coli K-12 genome suggest that the cloned gene is identical with gabT, a locus on the gab gene cluster of E. coli K-12 which codes for the GABA:2-ketoglutartate transaminase (EC 2.6.1.19). A number of expression plasmids carrying the isolated transaminase gene were constructed. With these constructs, the transaminase expression in transformants of E. coli could be increased up to 80-fold compared with that in a wild-type control, and the transaminase constituted up to 20% of the total soluble protein of the bacteria. Thus, the protein crude extracts of the transformants could be used, after a simple heat precipitation step, for the biotechnological production of L-phosphinothricin in an enzyme reactor.     

Stereospecific production of the herbicide phosphinothricin (glufosinate) by transamination: cloning, characterization, and overexpression of the gene encoding a phosphinothricin-specific transaminase from Escherichia coli

We have cloned the gene encoding a 43-kilodalton transaminase from Escherichia coli K-12 with a specificity for L-phosphinothricin [L-homoalanine-4-yl-(methyl)phosphinic acid], the active ingredient of the herbicide Basta (Hoechst AG). The structural gene was isolated, together with its own promoter, and shown to be localized on a 1.6-kilobase DraI-BamHI fragment. The gene is subject to catabolite repression by glucose; however, repression could be relieved completely when 4-aminobutyrate (GABA) served as the sole nitrogen source. The regulation pattern obtained and a comparison of the restriction map of the initially cloned 15-kilobase SalI fragment with the physical map of the E. coli K-12 genome suggest that the cloned gene is identical with gabT, a locus on the gab gene cluster of E. coli K-12 which codes for the GABA:2-ketoglutartate transaminase (EC 2.6.1.19). A number of expression plasmids carrying the isolated transaminase gene were constructed. With these constructs, the transaminase expression in transformants of E. coli could be increased up to 80-fold compared with that in a wild-type control, and the transaminase constituted up to 20% of the total soluble protein of the bacteria. Thus, the protein crude extracts of the transformants could be used, after a simple heat precipitation step, for the biotechnological production of L-phosphinothricin in an enzyme reactor.     

Cloning of the Escherichia coli K-12 dihydrofolate reductase gene following Mu-mediated transposition

The dihydrofolate reductase structural gene, folA, has been cloned into the multicopy vector pBR322 following the gene's enrichment by bacteriophage Mu-mediated transposition. Strains carrying the resultant plasmid, pJFMS, produce 25 to 30 times more dihydrofolate reductase than control strains. Consequently they are resistant to trimethoprim, an inhibitor of this enzyme. This elevation in enzyme production is due to an increase in the number of folA gene copies per cell. The higher yield of dihydrofolate reductase obtained will be extremely useful for purifying and characterising this trimethoprim-sensitive chromosomally derived enzyme. The plasmid will also be invaluable for studying the structure, function and regulation of dihydrofolate reductase.     

The purification and properties of the trimethoprim-resistant dihydrofolate reductase mediated by the R-factor, R388.

The R-factor R388 mediates the production of a trimethoprim-resistant dihydrofolate reductase. This enzyme has a different molecular weight and pH profile to the trimethoprim-sensitive enzyme of the Escherichia coli host. The R-factor mediated enzyme was separated completely from the host E. coli enzyme by DEAE-cellulose ion-exchange chromatography. The purified R-factor enzyme was about 20 000 times less susceptible to trimethoprim than the E. coli enzyme and although it was inhibited competitively by trimethoprim, its inhibitor constant (Ki) was 20 000 times greater than that of the host enzyme. The R388 and E. coli enzymes also differed in their substrate specificity requirements. In addition, the R388 enzyme suprisingly conferred high level resistance to the broad spectrum dihydrofolate reductase inhibitor, amethopterin. The possible origins of the R388 enzyme are discussed.     

Cloning and overproduction of biodegradative threonine deaminase from Escherichia coli W strain.

We have cloned the structural gene (tdcB) of biodegradative threonine deaminase from Escherichia coli W strain by utilizing the polymerase chain reaction. The JM109/pUCTDA strain, which was obtained by transforming E. coli JM109 with a vector plasmid (pUCTDA) containing the cloned tdcB gene, produced a large amount of the enzyme corresponding to more than 5% of the total soluble protein. Amino acid sequence analysis of this recombinant enzyme showed that the amino acid sequence is identical to the nucleotide-deduced sequence of biodegradative threonine deaminase from E. coli K-12.     

Trimethoprim resistance plasmids of different origin encode different drug-resistant dihydrofolate reductases.

Several plasmids mediating resistance to folic acid analogs were studied. The plasmids were in part newly isolated from clinical material and in part R factors studied earlier, such as R483, R721, R751, and R388. By gel chromatography, plasmid-carrying bacterial strains were all found to produce drug-resistant dihydrofolate reductases of a molecular weight distinctly larger than that of the chromosomal enzyme of the host. By gel electrophoresis and zymographic detection technique, analog inhibition characteristics, heat sensitivity, and pH optimum curves, the dihydrofolate reductases induced by R483, R751, and R388, respectively, could be clearly discerned as separate enzymes. Of the newly isolated plasmids all but one coded for a dihydrofolate reductase similar to that of R483. The aberrant one seemed to yield a new enzyme variant as judged from its drug inhibition characteristics and its pH optimum profile. Large differences in drug insensitivity were observed, thus the R751 and R388 enzymes were virtually insensitive to folic acid analogs, whereas the corresponding enzymes from the newly isolated plasmids, and from R483 showed a substantially higher sensitivity. On the other hand these latter enzymes were overproduced, in that the plasmid-carrying bacteria showed a 10- to 20-fold higher content of dihydrofolate reductase than the plasmid-free host strain. Among newly isolated trimethoprim-resistant strains, one was found which overproduced dihydrofolate reductase about 200-fold. In this case the enzyme was only slightly more resistant to folic acid analogs than the chromosomal Escherichia coli K-12 enzyme, and did not seem to be plasmid borne.     

Cloning and expression of the Escherichia coli glgC gene from a mutant containing an ADPglucose pyrophosphorylase with altered allosteric properties.

A mutant strain of Escherichia coli K-12, designated 618, accumulates glycogen at a faster rate than wild-type strain 356. The mutation affects the ADPglucose pyrophosphorylase regulatory properties (N. Creuzat-Sigal, M. Latil-Damotte, J. Cattaneo, and J. Puig, p. 647-680, in R. Piras and H. G. Pontis, ed., Biochemistry of the Glycocide Linkage, 1972). The enzyme is less dependent on the activator, fructose 1,6 bis-phosphate for activity and is less sensitive to inhibition by the inhibitor, 5'-AMP. The structural gene, glgC, for this allosteric mutant enzyme was cloned into the bacterial plasmid pBR322 by inserting the chromosomal DNA at the PstI site. The glycogen biosynthetic genes were selected by cotransformation of the neighboring asd gene into an E. coli mutant also defective in branching enzyme (glgB) activity. Two recombinant plasmids, pEBL1 and pEBL3, that had PstI chromosomal DNA inserts containing glgC and glgB were isolated. Branching enzyme and ADPglucose pyrophosphorylase activities were increased 240- and 40-fold, respectively, in the asd glgB mutant, E. coli K-12 6281. The E. coli K-12 618 mutant glgC gene product was characterized after transformation of an E. coli B ADPglucose pyrophosphorylase mutant with the recombinant plasmid pEBL3. The kinetic properties of the cloned ADPglucose pyrophosphorylase were similar to those of the E. coli K-12 618 enzyme. The inserted DNA in pEBL1 was arranged in opposite orientation to that in pEBL3.     

Resistance of Pediococcus cerevisiae to amethopterin as a consequence of changes in enzymatic activity and cell permeability. I. Dihydrofolate reductase, thymidylate synthetase and formyltetrahydrofolate synthetase in amethopterin-resistant and -sensitive strains of Pediococcus cerevisiae.

Pediococcus cerevisiae/AMr, resistant to amethopterin, possesses a higher dihydrofolate reductase (5, 6, 7, 8-tetrahydrofolate: NADP+ oxidoreductase, EC 1.5.1.3) activity than the parent, a folate-permeable and thus amethopterin-susceptible strain and than the wild-type. The properties of dihydrofolate reductase from the three strains have been compared. Temperature, pH optima, heat stability, as well amethopterin binding did not reveal significant differences between the enzymes from the susceptible and resistant strains. The enzyme from the wild-type was 10 times more sensitive to inhibition by amethopterin and more susceptible to heat denaturation. The apparent Km values for dihydrofolate in enzymes from the three strains were in the range of 4.8--7.2 muM and for NADPH 6.5--8.0 muM. The amethopterin-resistant strain exhibited cross-resistance to trimethoprim and was about 40-fold more resistant to the latter than the sensitive parent and the wild-type. The resistance to trimethoprim appears to be a direct result of the increased dihydrofolate reductase activity. Inhibition of dihydrofolate reductase activity by this drug was similar in the three strains. 10--20 nmol caused 50% inhibition of 0.02 enzyme unit. Trimethoprim was about 10 000 times less effective inhibitor of dihydrofolate reductase than amethopterin. The cell extract of the AMr strain possessed a folate reductase activity three times higher than that of the sensitive strain. The activities of other folate-related enzymes like thymidylate synthetase and 10-formyltetrahydrofolate synthetase (formate: tetrahydrofolate ligase (ADP-forming), EC 6.3.4.3) were similar in the three strains studied.