The basis for half-site specificity explored through a non-cognate steroid receptor-DNA complex.

Steroid receptors recognize bipartite targets composed of six base-pair half-sites. There are two canonical types of half-site which differ only in their central two base pairs. The crystal structure of an estrogen receptor-like DNA-binding domain bound to the wrong type of half-site (a glucocorticoid response element) reveals an interface that resembles the specific interfaces of the glucocorticoid receptor or estrogen receptor bound to their correct response elements. The underlying stereochemical defect that weakens the non-cognate interface is a difference in the helical geometry of the incorrect DNA half-site which prevents a side-chain contact and results in a gap which is filled by at least five additional fixed water sites, imposing a potential entropic burden on the stability of the interface.