Does the channel for nascent peptide exist inside the ribosome? Immune electron microscopy study

Abstract

MS2 phage RNA‐directed synthesis of an N‐terminal polypeptide of the phage coat protein on Escherichia coli 70 S ribosomes was initiated in a cell‐free system with the N‐dinitrophenyl derivative of methionyl‐tRNA^Met _F and performed in the absence of tyrosine, lysine, cysteine and methionine. As a result, the translating ribosomes carried peptides up to 42 amino acid residues in length with the dinitrophenyl hapten at the N‐ends. Using the immune electron microscopy technique the positions of the nascent peptide N‐ends on the 70 S ribosomes have been visualized. It has been found that (i) the N‐ends of nascent peptides of these lengths are accessible to antibodies, (ii) the exit site of a nascent peptide is the pocket between the base of the central protuberance and the L1 ridge on the 50 S subunit, i.e. presumably its peptidyl transferase center, and (iii) the further pathway of a nascent peptide seems to proceed along the groove on the external surface of the 50 S subunit.