The Escherichia coli adhesin F17-G is a carbohydrate-binding protein that allows the bacterium to attach to the intestinal epithelium of young ruminants. The structure of the 17 kDa lectin domain of F17-G was detd. using the anomalous dispersion signal of a selenium-contg. analog of the monosaccharide ligand N-acetyl-D-glucosamine in which the anomeric oxygen was replaced by an Se atom. A three-wavelength MAD data set yielded good exptl. phases to 2.6 .ANG. resoln. The structure was refined to 1.75 .ANG. resoln. and was used to solve the structures of the ligand-free protein and the F17-G-N-acetyl-D-glucosamine complex. This selenium-carbohydrate phasing method could be of general use for detg. the structures of carbohydrate-binding proteins. [on SciFinder (R)]