Glycosyltransferases are enzymes responsible for the transfer of sugar residues to a wide variety of acceptors. This is the most abundant reaction on the Earth in massterms. We particularly use prokaryotic glycosyltransferases that participate in the biosynthesis of bacterial polysaccharides. We tried to know in detail the molecular bases of the transfer reaction with the purpose of being able to control these reactions. We have purified two glycosyltransferases associated to the cytoplasmic membrane. AceA is a mannosyltransferase of Acetobacter xylinum that takes part in the acetan polysaccharide biosynthesis. GumK is a glucuronosyltransferase of Xanthomonas campestris involved in the synthesis of xanthan, a polysaccharide with many industrial applications. Right now, the structural bases that determine the function of these enzymes and the recognition by their substrates are barely known. To elucidate the molecular function of these enzymes we have collected x-ray diffraction data from protein crystals, by themselves and in complex with one of its two substrates. In this way we deciphered the molecular structure of GumK at 1,9 Å resolution. The structure of this enzyme is arranged in 2 globular domains connected by a linker loop. This loop is the bottom of a deep cleft that separates both domains. The cleft has a maximum width of 25 Å and a depth of 15 Å. It is in a high specific pocket of this cleft where we observed the binding site of the enzyme's substrate donor. At the moment we are complementing these structural studies with studies of site-directed mutagenesis on amino acids involved in the binding of substrates or catalysis. Our objective is to try to understand in greater detail the molecular operation of these enzymes, to modify the preferences of the enzyme by its substrates, and possibly to extend this knowledge to other related enzymes. This knowledge could be very important to biotechnology.