title: Substrate and DNA binding to a 50-residue peptide fragment of DNA polymerase I. Comparison with the enzyme.
abstract: The fluorescent nucleotide 2',3'-trinitrophenyl-ATP (TNP-ATP) binds at the triphosphate substrate binding site of the large (Klenow) fragment of DNA polymerase I (Pol I) as detected by direct binding studies measuring the increase in fluorescence of this ligand (n = 1.0, KD = 0.07 microM). The enzyme-TNP-ATP complex binds Mg2+ and Mn2+ tightly (KD = 0.05 microM) as measured by an increase in fluorescence on titrating with these metals. The substrate dGTP competitively displaces TNP-ATP from the enzyme (KD = 5.7 microM) de-enhancing the fluorescence. The polymerase reaction is half-maximally inhibited by 0.8 microM TNP-ATP in the presence of dATP (10 microM) as substrate. A region of the amino acid sequence of Pol I (peptide I) consisting of residues 728-777 has been synthesized and found to contain significant secondary structure by CD both in water and 50% methanol/water. In water at 3 degrees C, peptide I binds the substrate analog TNP-ATP (KD = 0.03 microM) with a stoichiometry of 0.2. In 50% methanol at 3 degrees C, peptide I binds TNP-ATP with a higher stoichiometry than in water, consistent with a 1:1 complex, but biphasically (16% of the peptide, KD = 0.09 microM; 84% of the peptide, KD = 5.0 microM), and competitively binds the Pol I substrates dATP, TTP, and dGTP (KD = 230-570 microM). Evidence from size exclusion high performance liquid chromatography suggests that these two forms of the peptide are monomer and dimer, respectively. Significantly, the peptide I-TNP-ATP complex binds duplex DNA, tightly (KD = 0.1-0.5 microM) and stoichiometrically, and single stranded DNA more weakly. The peptide I-duplex DNA complex binds both TNP-ATP (KD = 0.5-1.5 microM) and Pol I substrates (KD = 350-2100 microM) stoichiometrically. In a control experiment, a second peptide, peptide II, based on residues 840-888 of the Pol I sequence, retains secondary structure, as detected by CD, but displays no binding of TNP-ATP. The ability of peptide I, which represents only 8% of the large fragment of Pol I, to bind both substrates and duplex DNA indicates that residues 728-777 constitute a major portion of the substrate binding site of this enzyme.
authors.affiliation: Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
authors.forename: G P
authors.initials: GP
authors.lastname: Mullen
authors.forename: P
authors.initials: P
authors.lastname: Shenbagamurthi
authors.forename: A S
authors.initials: AS
authors.lastname: Mildvan
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology / Q000031:analogs & derivatives; D000595:Amino Acid Sequence; D002942:Circular Dichroism; D004247:DNA / Q000378:metabolism*; D004256:DNA Polymerase I / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004926:Escherichia coli / Q000201:enzymology; D005456:Fluorescent Dyes; D007700:Kinetics; D008433:Mathematics; D008961:Models, Structural; D008962:Models, Theoretical; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000302:isolation & purification / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence
chemical_list: C009587:2',3'-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate; D004268:DNA-Binding Proteins; D005456:Fluorescent Dyes; D010446:Peptide Fragments; D000255:Adenosine Triphosphate; D004247:DNA; D004256:DNA Polymerase I