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.