The GumH enzyme from Xylella fastidiosa catalyzes the transfer reaction of a mannose from GDP-mannose to the carrier lipid cellobiose-pyrophosphate-polyprenol (GIc(2)-PP-Lip), an intermediary in the reaction for the synthesis of the exopolysaccharide (EPS) fastidian gum. The gumH gene was subcloned in the pMal-c2x vector, allowing the expression of the GumH-MBP fusion protein. Various attempts were made to obtain protein with the necessary degree of purity for crystallographic studies but the yield was very low. The gumH gene was then subcloned in the pET28a vector allowing the expression of the GumH enzyme in fusion with a histidine-rich peptide. The protein was purified and characterized. The three-dimensional structure of the X. fastidiosa GumH enzyme was modeled by threading studies. The model consists of N- and C-terminal domains similar in size and topology and separated by a deep cleft, which includes the EX7E motif that can be involved in the catalysis of GumH. (C) 2004 Elsevier Inc. All rights reserved.