Glycosyl-1-phosphate transferase that mediates the first step in the biosynthesis of the undecaprenyl-linked heptasaccharide donor in the N-linked protein glycosylation pathway. Catalyzes the linking of uridine 5'-diphosphobacillosamine (UDP-Bac) to undecaprenyl phosphate to create the first membrane-associated intermediate undecaprenylpyrophosphate-linked Bac (Und-PP-Bac). tri-trans,hepta-cis-undecaprenyl phosphate + UDP-N,N'-diacetylbacillosamine = N,N'-diacetyl-alpha-D-bacillosaminyl-tri-trans,hepta-cis-undecaprenyl diphosphate + UMP Protein modification; protein glycosylation. N-linked protein glycosylation in C.jejuni consists in the transfer of a heptasaccharide (GalNAc-alpha1,4-GalNAc-alpha1,4-(Glcbeta1,3)-GalNAc-alpha1,4-GalNAc-alpha1,4-GalNAc-alpha1,3-bacillosamine) from a membrane-anchored undecaprenylpyrophosphate (Und-PP)-linked donor to the Asn side chain of proteins at the Asn-X-Ser/Thr motif. Belongs to the bacterial sugar transferase family.