The influence of tryptophan on mobility of residues in the trp repressor of Escherichia coli

The relative mobility of residues in the trp repressor of Escherichia coli has been examined in the absence and presence of the corepressor l‐tryptophan by one‐ and two‐dimensional ¹H NMR. A comparison of relative intensities of cross peaks in NOESY and COSY spectra allowed a rigid Tyr and a mobile Tyr residue, three mobile Ser residues and three mobile Lys residues to be detected. The two Tyr residues were assigned by selective nitration with tetranitormethane. The singly nitrated molecule (on Tyr7) binds the trp operator with an affinity close to that of the unmodified repressor. Measurements of the intraring cross‐relaxation rate constant as a function of temperature for Tyr7 shows the presence of considerable internal motion on the subnanosecond time scale in the flexible N‐terminal arm. The order parameter, S² characterising the motion is 0.35, which increases to about 0.5 in the presence of Trp. Trp decreases both the amplitude of the motion and the rate of the motion. At least three of the six Ser residues of the trp repressor have greater mobility than expected for a rigid body, and two of the Ser residues are sensitive to the presence of Trp. The more mobile Ser residues are probably those on the N‐terminal arm and the C‐terminal sequence. These results complement the single‐crystal X‐ray diffraction studies for which the electron density of the first ten and last three amino acid residues is weak. The solution data are consistent with proposals that the flexible N‐terminal arm of the trp repressor makes important contacts with the DNA.