Oligosaccharyl transferase (OST) that catalyzes the initial transfer of a defined glycan (GalNAc(2)GlcGalNAc(3)Bac(NAc)(2) in eubacteria, where Bac(NAc)(2) is di-N-acetyl bacillosamine) from the lipid carrier undecaprenol-pyrophosphate to an asparagine residue within an Asp/Glu-Asn-X-Ser/Thr consensus motif in nascent polypeptide chains, the first step in protein N-glycosylation. tritrans,heptacis-undecaprenyl diphosphooligosaccharide + [protein]-L-asparagine = tritrans,heptacis-undecaprenyl diphosphate + a glycoprotein with the oligosaccharide chain attached by N-beta-D-glycosyl linkage to protein L-asparagine. Glycosylates at a rate of 1.55 peptides per second per OST. Optimum pH is 6.8. Protein modification; protein glycosylation. Despite low primary sequence conservation between eukaryotic catalytic subunits and bacterial and archaeal single subunit OSTs (ssOST), structural comparison revealed several common motifs at spatially equivalent positions, like the DXD motif 1 on the external loop 1 and the DXD motif 2 on the external loop 2 involved in binding of the metal ion cofactor and the carboxamide group of the acceptor asparagine, the conserved Glu residue of the TIXE/SVSE motif on the external loop 5 involved in catalysis, as well as the WWDYG and the DK/MI motifs in the globular domain that define the binding pocket for the +2 Ser/Thr of the acceptor sequon. In bacterial ssOSTs, an Arg residue was found to interact with a negatively charged side chain at the -2 position of the sequon. This Arg is conserved in bacterial enzymes and correlates with an extended sequon requirement (Asp-X-Asn-X-Ser/Thr) for bacterial N-glycosylation. Belongs to the STT3 family.