15N-labeled P22 c2 repressor for nuclear magnetic resonance studies of protein-DNA interactions.

The salmonella phage P22 c2 repressor was produced with 90% 15N isotope labeling of all leucines, using the expression system E. coli W3110 lac IQ¿P 125. The N-terminal DNA-binding domain 1-76 was obtained by chymotrypsin cleavage. Its characterization by biochemical techniques, mass spectrometry, and one- and two-dimensional nuclear magnetic resonance (NMR) showed that highly residue-selective isotope labeling was achieved with the minimal growth medium used. The ability to obtain such isotope labeling opens new avenues for NMR studies of protein-DNA interactions in the P22 operator system.