On the Role of the Starved Codon and the Takeoff Site in Ribosome Bypassing in Escherichia coli

Translating ribosomes can skip over stretches of messenger RNA and resume protein chain elongation after a “bypassed” region. We have previously shown that limitation for isoleucyl-tRNA can initiate a ribosome bypass when an AUA codon is in the ribosomal A-site. We have now generalized this effect to other “hungry” codons calling for four different limiting aminoacyl-tRNA species, suggesting that a pause at any A-site will have this effect. We have assessed bypassing in a large family of reporters with nearly every different triplet in the “takeoff site”, i.e. the P-site on the 5′ side of the hungry codon, and an identical “landing site” codon 16 nucleotides downstream. The different takeoff sites vary over a factor of 50 in bypassing proficiency. At least part of this variation appears to reflect stability of the codon∷anticodon interaction at the takeoff site, as indicated by the following: (a) the bypassing proficiency of different tRNAs shows a rough correlation with the frequency of A∷U as opposed to G∷C pairs in the codon∷anticodon association; (b) specific tRNAs bypass more frequently from codons ending in U than from their synonym ending in C; (c) an arginine tRNA with Inosine in the wobble position which reads CGU, CGC, and CGA bypasses much more frequently from the last codon than the first two synonyms.