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Abstract

During translation, usually only one in approximately 400 misincorporations affects the function of a nascent protein, because only chemically similar near-cognate amino acids are misincorporated in place of the cognate one. The deleterious misincorporation of a chemically dissimilar noncognate amino acid during the selection process is precluded by the presence of a tRNA at the ribosomal E-site. However, the selection of first aminoacyl-tRNA, directly after initiation, occurs without an occupied E-site, i.e., when only the P-site is filled with the initiator tRNA and thus should be highly error-prone. Here, we show how bacterial ribosomes have solved this accuracy problem: In the absence of a Shine-Dalgarno (SD) sequence, the first decoding step at the A-site after initiation is extremely error-prone, even resulting in the significant incorporation of noncognate amino acids. In contrast, when a SD sequence is present, the incorporation of noncognate amino acids is not observed. This is precisely the effect that the presence of a cognate tRNA at the E-site has during the elongation phase. These findings suggest that during the initiation phase, the SD interaction functionally compensates for the lack of codon-anticodon interaction at the E-site by reducing the misincorporation of near-cognate amino acids and prevents noncognate misincorporation.