MAPPING THE PATH OF THE NASCENT PEPTIDE-CHAIN THROUGH THE 23S RNA IN THE 50S RIBOSOMAL-SUBUNIT

Peptides of different lengths encoded by suitable mRNA fragments were biosynthesized in situ on Escherichia coli ribosomes, The peptides carried a diazirine derivative bound to their N-terminal methionine residue, which was photoactivated whilst the peptides were still attached to the ribosome, Subsequently, the sites of photo-cross-linking to 23S RNA were analyzed by our standard procedures, The N-termini of peptides of increasing length became progressively cross-linked to nucleotide 750 (peptides of 6, 9 or 13-15 amino acids), to nucleotide 1614 and concomitantly to a second site between nucleotides 1305 and 1350 (a peptide of 25-26 amino acids), and to nucleotide 91 (a peptide of 29-33 amino acids), Previously we had shown that peptides of 1 or 2 amino acids were cross-linked to nucleotides 2062, 2506 and 2585 within the peptidyl transferase ring, whereas triand tetrapeptides were additionally cross-linked to nucleotides 2609 and 1781, Taken together, the data demonstrate that the path of the nascent peptide chain moves from the peptidyl transferase ring in domain V of the 23S RNA to domain IV, then to domain II, then to domain III, and finally to domain I, These cross-linking results are correlated with other types of topographical data relating to the 50S subunit.