Nonprocessive beta-mannosyltransferase that catalyzes the transfer of a mannose residue from GDP-mannose to glucuronic acid-beta-1,2-mannose-alpha-1,3-glucose-beta-1,4-glucose-PP-polyisoprenyl to form the lipid-linked pentasaccharide repeating unit of xanthan, Man-GlcA-Man-Glc(2)-PP-Pol. Is involved in the biosynthesis of the exopolysaccharide xanthan. To a lesser extent, can also use ADP-Man and even GDP-Glc as sugar donor substrates in vitro. Is unable to transfer a Man residue to the free-tetrasaccharide GlcA-Man-Glc(2) used as an acceptor, which indicates that the diphosphate group and the lipid moiety in the acceptor substrate are of major importance for acceptor binding and catalysis. beta-D-GlcA-(1->2)-alpha-D-Man-(1->3)-beta-D-Glc-(1->4)-alpha-D-Glc-di-trans,octa-cis-undecaprenyl diphosphate + GDP-alpha-D-mannose = beta-D-Man-(1->4)-beta-D-GlcA-(1->2)-alpha-D-Man-(1->3)-beta-D-Glc-(1->4)-alpha-D-Glc-di-trans,octa-cis-undecaprenyl diphosphate + GDP + H(+) Optimum pH is around 8. Activity decreases below pH 7.5 and above pH 9.5. Optimum temperature is aroud 35 degrees Celsius with a 20% drop at 20 degrees Celsius and a near-complete inhibition at 60 degrees Celsius, probably due to enzyme denaturation. Glycan biosynthesis; xanthan biosynthesis. Monotopic integral membrane protein, mostly cytoplasmic. Cells lacking this gene display a non-mucoid phenotype, indicating that they have a reduced abundance of polysaccharides relative to the wild-type strain. More precisely, mutational interruption of the gumI gene leads to accumulation of GlcA-beta-1,2-Man-alpha-1,3-Glc-beta-1,4-Glc-PP-polyisoprenyl (GlcA-Man-Glc2-PP-Pol), and a decrease in xanthan production of more than 95%. Belongs to the glycosyltransferase 94 family.