title: Purification and characterization of trp aporepressor.
abstract: We have isolated homogeneous trp aporepressor from an overproducing strain of Escherichia coli carrying a plasmid containing trpR preceded by tandem trp operon promoters. Dye-affinity and ion-exchange chromatography were used in conjunction with a gel electrophoresis assay in which the repressor, when bound to the trp operator, protects an Rsa I restriction site from endonuclease cleavage. Crystals suitable for x-ray diffraction studies were grown from a variety of concentrated salt solutions. Hydrodynamic properties and electrophoretic analysis of unmodified and covalently crosslinked aporepressor show that the free aporepressor has an isoelectric point of 5.9 and is a dimer containing two identical 12.5-kilodalton subunits in the presence or absence of L-tryptophan. The repressor . operator complex binds poorly to nitrocellulose filters, but restriction-site protection studies indicate that, in the presence of tryptophan, one dimer is bound to the operator site with an apparent dissociation constant less than 2 X 10(-9) M. Preliminary equilibrium dialysis experiments suggest that tryptophan binds to the aporepressor with a dissociation constant of 1.6 X 10(-5) M.
mesh_terms: D000595:Amino Acid Sequence; D001059:Apoproteins / Q000302:isolation & purification; D003461:Crystallography; D004926:Escherichia coli / Q000235:genetics; D005786:Gene Expression Regulation; D007526:Isoelectric Point; D008970:Molecular Weight; D009876:Operon; D011485:Protein Binding; D012097:Repressor Proteins / Q000302:isolation & purification* / Q000378:metabolism; D013057:Spectrum Analysis; D014157:Transcription Factors / Q000302:isolation & purification*; D014364:Tryptophan / Q000235:genetics* / Q000378:metabolism
chemical_list: D001059:Apoproteins; D012097:Repressor Proteins; D014157:Transcription Factors; D014364:Tryptophan