THE crystal structure of trp represser tandemly bound in a 2:1 complex to a 16-base-pair palindromic DNA containing a central trp operator half-site has been determined and refined to 2.4 & Aring; resolution. Despite dramatically different DNA sequence contexts and crystallization conditions, the protein/DNA interface is essentially identical to that seen in the original trp represser/operator complex structure1. Water-mediated sequence recognition by trp represser is likely to be related to the unusual end-on approach of the recognition helix (E), which allows sharing of the major groove by tandem dimers. The tandem complex model accounts for the mutational sensitivity of all trp operator base pairs. The structure also provides the first detailed view of the tandem interaction, revealing a key role for the amino-terminal arms.