Is the initiating enzyme for the synthesis of the exopolysaccharide xanthan. Catalyzes the transfer of the glucose-1-phosphate moiety from UDP-Glc onto the carrier lipid undecaprenyl phosphate (C55-P), forming a phosphoanhydride bond yielding to glucosyl-pyrophosphoryl-undecaprenol (Glc-PP-C55). di-trans,octa-cis-undecaprenyl phosphate + UDP-alpha-D-glucose = alpha-D-glucosyl di-trans,octa-cis-undecaprenyl diphosphate + UMP Glycan biosynthesis; xanthan biosynthesis. The C-terminal domain is sufficient for glucosyl-1-phosphate transferase activity. Cells lacking this gene fail to synthesize any xanthan biosynthetic lipid sugar intermediates. They do not produce xanthan and exhibit a 50% virulence index reduction. Belongs to the bacterial sugar transferase family.