Determination of the minimum number of residues required to span the distance between the ribosomal P-site and the active site of oligosaccharide transferase (dP-O values). Ribosome-bound truncated nascent chains of different lengths are generated by in vitro translation, in the presence of dog pancreas microsomes, of mRNAs lacking a stop-codon. The dP-O value is the number of residues between the Asn residue in an Asn-Ser-Thr glycosylation acceptor site (Y) and the C-terminal end of the nascent chain required for half-maximal glycosylation of the acceptor site. (A) The model protein used in this study (E. coli Lep) has two transmembrane segments (black) and a C-terminal lumenal domain. The approximate number of residues in an extended chain needed to span the indicated distances are given . (B) Truncated Lep with a model segment (white) placed in the ribosome. The glycosylation site is placed in position 178 in Lep. (C) Truncated Lep with a model segment (white) placed in the translocon. The glycosylation site is placed in position 200 in Lep. In both panels B and C, the model segment is introduced between positions 214 and 220 in Lep.