Direct recognition of the trp operator by the trp holorepressor--a review.

Two different X-ray co-crystal structures of the Escherichia coli trp holorepressor complexed with DNA suggest that the TrpR protein recognizes specific DNA sequences primarily with a network of water-mediated H-bonds. However, the more recent nuclear magnetic resonance (NMR) solution structures of the holorepressor-operator complex show no long-lived, ordered water molecules at the protein-DNA interface and place amino acids in intimate contact with nucleotide bases. Both genetic and biochemical studies support a model in which the trp repressor recognizes specific DNA sequences by a direct mechanism, as seen in the NMR solution structures, not by the 'indirect readout' mechanism initially proposed on the basis of X-ray studies.