Model for programmed translational bypassing in T4 gene 60. A) In most T4-related phages, the large topoisomerase subunit (gp39) is encoded by a single gene. In T4, active site domains are interrupted by the mobA-60.1 coding region where the downstream ORF (gp60) contains the 50 nucleotide bypassed sequence. Elements in the mRNA that code for translational bypassing are shown. B) Recapitulation of the model  where E-, P- and A- sites of the ribosome are shown as translation enters the bypass region (blue mRNA). Nascent peptide is shown in gold and as promoting "take-off" from the green GGA codon. In (C) the bypass mRNA stem-loop structure occupies the A-site, precluding release factor 1  from binding and inhibiting termination. Ribosomal protein L9 facilitates tRNA exit through the E-site and nascent peptide interactions prevent peptidyl-tRNA scanning. D) Landing site codon:anticodon pairing is shown, with entry of the resuming charged tRNA into the A-site for translating the distal region of gene 60 mRNA. The GAG six nucleotides 5' of the landing site is shown pairing with the 16S rRNA anti-SD region to help reinitiate peptidyl-tRNA scanning and pairing at the GGA. Figure adapted from , with kind permission of J. F. Atkins.