GlcA-beta-(1->2)-D-Man-alpha-(1->3)-D-Glc-beta-(1->4)-D-Glc-alpha-1-diphospho-ditrans,octacis-undecaprenol 4-beta-mannosyltransferase (, GumI) is an enzyme with systematic name GDP-mannose:GlcA-beta-(1->2)-D-Man-alpha-(1->3)-D-Glc-beta-(1->4)-D-Glc-alpha-1-diphospho-ditrans,octacis-undecaprenol 4-beta-mannosyltransferase. This enzyme catalyses the following chemical reaction
The enzyme is involved in the biosynthesis of the exopolysaccharide xanthan.