title: Characterization of an R-plasmid dihydrofolate reductase with a monomeric structure.
abstract: 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.
mesh_terms: D000595:Amino Acid Sequence; D002876:Chromosomes, Bacterial; D004926:Escherichia coli / Q000201:enzymology*; D007700:Kinetics; D008970:Molecular Weight; D011815:R Factors; D012483:Salmonella Phages / Q000201:enzymology; D012486:Salmonella typhimurium / Q000201:enzymology*; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism
chemical_list: D013762:Tetrahydrofolate Dehydrogenase