Is the cellular initiation of translation an exclusive property of the initiator tRNAs?

Abstract

Translation of mRNAs is the primary function of the ribosomal machinery. Although cells allow for a certain level of translational errors/mistranslation (which may well be a strategic need), maintenance of the fidelity of translation is vital for the cellular function and fitness. The P-site bound initiator tRNA selects the start codon in an mRNA and specifies the reading frame. A direct P-site binding of the initiator tRNA is a function of its special structural features, ribosomal elements, and the initiation factors. A highly conserved feature of the 3 consecutive G:C base pairs (3 GC pairs) in the anticodon stem of the initiator tRNAs is vital in directing it to the P-site. Mutations in the 3 GC pairs diminish/abolish initiation under normal physiological conditions. Using molecular genetics approaches, we have identified conditions that allow initiation with the mutant tRNAs in Escherichia coli. During our studies, we have uncovered a novel phenomenon of in vivo initiation by elongator tRNAs. Here, we recapitulate how the cellular abundance of the initiator tRNA, and nucleoside modifications in rRNA are connected with the tRNA selection in the P-site. We then discuss our recent finding of how a conserved feature in the mRNA, the Shine-Dalgarno sequence, influences tRNA selection in the P-site.

Structure of a 30S pre-initiation complex stalled by GE81112 reveals structural parallels in bacterial and eukaryotic protein synthesis initiation pathways

“…The cryo-EM density maps also indicate that fMet–tRNA accommodation into the P IN site is accompanied by a closure of the 30S head around the fMet–tRNA (Figure 2A ; red arrow) so that G1338 and A1339 of 16S rRNA could form the characteristic A-minor interactions with the G-C base pairs of initiator tRNA anticodon stem only with the P IN tRNA (Figure 2D and E ). This interaction was shown to increase the stability of the complexes containing P-site bound fMet–tRNA ( 31 , 32 ). In yeast equivalent interactions are made between conserved G-C pairs in the anticodon stem–loop of yeast initiator tRNA and 18S rRNA residues (G1575 and A1576) and presumably stabilize the P IN state ( 25 , 33 , 34 ).…”

Measurements of translation initiation from all 64 codons in E. coli

“…The reported rate of base misincorporation during transcription ranges between 10 −4 ( 70 ) and 10 −5 per base ( 71 ), although in a codon that is one base away from a canonical start codon, only one out of three random mutations would result in an AUG start codon. Estimates for the rate of elongator tRNA misacylation range between 10 −4 ( 68 ) and 10 −6 ( 72 ), although these rates would need to be compounded with the likelihood of the mischarged amino acid being methionine ( 73 ), and discrimination by IF3 in the ribosomal P-site ( 74 – 77 ) for the stem-loop structure of initiator tRNAs ( 78 , 79 ). The rate of mRNA codon misreading (i.e.…”

Faculty Opinions recommendation of Measurements of translation initiation from all 64 codons in E. coli.

“…The reported rate of base misincorporation during transcription ranges between 10 −4 (70) and 10 −5 per base (71), although in a codon that is one base away from a canonical start codon, only one out of three random mutations would result in an AUG start codon. Estimates for the rate of elongator tRNA misacylation range between 10 −4 (68) and 10 −6 (72), although these rates would need to be compounded with the likelihood of the mischarged amino acid being methionine (73), and discrimination by IF3 in the ribosomal P-site (74)(75)(76)(77) for the stem-loop structure of initiator tRNAs (78,79). The rate of mRNA codon misreading (i.e.…”