title: Identification of amino acids in lac repressor protein cross-linked to operator DNA specifically substituted with bromodeoxyuridine.
abstract: Amino acids in lac repressor protein which form cross-links to lac operator DNA specifically substituted with bromodeoxyuridine (BrdU) have been identified. Five sites of cross-linking in BrdU-substituted operator DNA were found at positions +3, +4, +14, +18, and +19 relative to the initiation site for transcription (Wick, K.L., and Matthews, K.S. (1991) J. Biol. Chem. 266, 6106-6112). Irradiation of complexes of repressor and each of these five singly substituted operator DNAs was executed under large scale conditions to generate sufficient complex for proteolysis, separation of the peptide-DNA, and peptide sequencing. The DNAs substituted with BrdU for thymidine at positions +3, +18, and +19 yielded cross-links to the peptide spanning residues 23-33, with the cross-link identified at His-29. Substitution at position +14 resulted in a cross-link to Tyr-17 within the peptide containing amino acids 13-22. These results are consistent with the structure determined by NMR and molecular dynamics calculations of the NH2-terminal headpiece-symmetric operator complex (Lamerichs, R.M.J.N., Boelens, R., van der Marel, G.A., van Boom, J.H., Kaptein, R., Buck, F., Fera, B., and Rüterjans, H. (1989) Biochemistry 28, 2895-2991; de Vlieg, J., Berendsen, H.J.C., and van Gunsteren, W.F. (1989) Proteins 6, 104-127). This structure indicates proximity of His-29 in the major groove to thymidines at positions +3 and +4. Since base pairs at positions +18 and +19 occupy symmetrical positions to +3 and +4 in the promoter distal region of the operator, it would be anticipated that cross-links similar to the +3 and +4 positions would form at these sites; this prediction is not borne out by the behavior at +4/+18, as no peptide could be identified cross-linked to DNA substituted at +4. Molecular dynamics simulations and the NMR data indicate that Tyr-17 interacts with the thymine at position +8, which is symmetrically related to position +14. Although BrdU-associated strand scission at +8 is protected, this site does not cross-link with bound lac repressor; inversely, DNA substituted with BrdU at +14 cross-links to repressor, but is not protected from strand scission by the presence of the protein. These differences at symmetrically related nucleotide positions (+4 versus +18, +8 versus +14) reflect the inherent asymmetry in the interaction. The identification of amino acids in proximity to specifically substituted sites confirms that positions of several amino acids in the intact protein-operator complex correspond to those in the structure of the NH2-terminal headpiece-operator DNA complex.
authors.affiliation: Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77251.
authors.forename: T D
authors.initials: TD
authors.lastname: Allen
authors.forename: K L
authors.initials: KL
authors.lastname: Wick
authors.forename: K S
authors.initials: KS
authors.lastname: Matthews
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis*; D001483:Base Sequence; D001973:Bromodeoxyuridine / Q000378:metabolism*; D002852:Chromatography, Ion Exchange; D003432:Cross-Linking Reagents; D004247:DNA / Q000235:genetics / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D006868:Hydrolysis; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D009875:Operator Regions, Genetic; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D003432:Cross-Linking Reagents; D012097:Repressor Proteins; D004247:DNA; D001973:Bromodeoxyuridine