An acyltransferase was detected in young leaves of pedunculate oak (Quercus robur) that catalyzed the formation of 1,2,3,4,6-penta-O-galloyl-beta-D-glucose, the common precursor of gallotannins and the related ellagitannins. This enzyme depended on beta-glucogallin (1-O-galloyl-beta-D-glucose) as acyl donor; 1,2,3,6-tetra-O-galloyl-beta-D-glucose was specifically required as acceptor molecule, whereas no reaction occurred with the 1,2,4,6-isomer of this substrate. The partially purified enzyme (Mr 260,000) was stable between pH 5.0 and 6.5; highest activities were observed at pH 6.3 and 40 degrees C. Km values of 2.3 and 1.0 mM, respectively, were determined for the substrates beta-glucogallin and tetragalloylglucose. In accordance with stoichiometric studies, the systematic name "beta-glucogallin: 1,2,3,6-tetra-O-galloylglucose 4-O-galloyltransferase" is proposed for this new enzyme.