O-antigen group O:21 polymerase yielding a long O antigen chain length (25 repeat units)
25.0 udcdpgalnac_b1_3_gal_a1_4_gal_b1_3_galnac_a1_3_glcnac_p ⇌ 24.0 udcpdp_p + OA21__L_p