lambda Phage cro repressor interaction with DNA.

We present here the complete identification of the resonances from the aromatic region of the 1H NMR spectrum of the cro repressor of the Escherichia coli lysogenic phage lambda. This was accomplished by the use of two-dimensional NMR analysis as well as specifically deuterated tyrosines. Not surprisingly, it shows that the published resonance assignment approached by more conventional methods by others includes substantial errors. The effect of complex formation with DNA was examined in the 1H NMR spectrum as well as in 19F NMR spectra from 3-fluorophenylalanine- or 3-fluorotyrosine-substituted cro repressors. The fluoro analogues show the practicality of using a structural gene cloned into an inducible system as a starting point to obtain both material and specific nuclear spin incorporation for NMR spectroscopy. The NMR data offer direct support for the speculative models of cro repressor-DNA complexes proposed from x-ray structure analysis on the protein alone.