Genetic insertion of nuclear spin labels in the lac repressor

A general strategy for the insertion of nuclear spin labels throughout the sequence of a protein is illustrated with the Escherichia coli lac repressor. Examples are shown where the 19F nucleus is incorporated using 3-fluorotyrosine, as well as the selective insertion of additional protons. These selectively inserted nuclei give additional resonances in the respective NMR spectra that can be used to probe the structure and function of the proteins.

Genetic assignment of resonances in the NMR spectrum of a protein: lac repressor

By using a systematic genetic approach, the resonances in the 19F NMR spectrum of 3-fluorotyrosine-substituted lac repressor protein have been assigned. The NMR data indicate that each monomer of the repressor consists of two distinct and independent domains. One domain, the NH2-terminal sixth of the primary sequence, which has been shown to be very important for DNA binding, is very mobile. The remaining COOH-terminal sequence is more rigid. Ligands of the repressor, which affect its DNA binding capability, lead to conformational changes in the COOH-terminal domain. The approach to the assignment of spectral features taken here can be extended to other systems.

lac repressor: 3-fluorotyrosine substitution for nuclear magnetic resonance studies

This paper describes the isolation of 3-fluorotyrosine-substituted lac repressor, and its 19F nuclear magnetic resonance spectrum. From the spectrum, one can conclude that for each of the four identical subunits of the repressor there are four or five surface tyrosines, two buried or internal tyrosines, and one tyrosine with an phenolic group ionized or involved in a hydrogen bond. Conditions are described that can be used for the 3-fluorotyrosine substitution of a variety of Escherichia coli proteins for 19F nuclear magnetic resonance studies.

Lac Repressor. Fluorescence of the two tryptophans

By comparing the fluorescence emission properties of the wild type lac repressor with two lac repressors altered at tryptophan 190 and 209, respectively, we show that tryptophan residue 209 has its environment changed, either by its own motion, or that of the surrounding amino acids when an inducer molecule is bound. Substitution of this tryptophan with other amino acids results in lac respressor molecules with reduced affinity for inducer molecules, indicating that the geometry at residue 209 affects the geometry of the inducer-binding site. From the results of potassium iodide quenching of fluorescence from the tryptophans, and from attempts to react the native lac repressor with dimethyl(2-hydroxy-5-nitrobenzyl) sulfonium bromide and 2-hydroxy-5-nitrobenzyl bromide, we propose that tryptophan residue 209 is involved in a conformational change of the protein upon binding of inducer, but does not come in direct contact with inducer.