![]() ![]() In addition to the invariant ATG codon, a considerable sequence homology is found around the 5′-untranslated region flanking the ATG codon. Although it has been generally believed that the translation initiation occurs at the ATG codon nearest the cap site, the downstream second ATG often functions. In the process of translation of eukaryotic mRNAs, the 40S ribosomal subunits appear to bind first at the 5′-end (cap site) of mRNA and to scan the mRNA until the subunits find an AUG translation initiation codon (hereafter, U is described by T). Her experimental results were well understood on the basis of the strength of the translation initiation signal sequence. So far, Kozak has constructed a number of preproinsulin mutant mRNAs in which nucleotides flanking the ATG codon are systematically varied, and measured the translation initiation efficiency in terms of the proinsulin product. ![]() In this method, each 16 nt sequence in the mRNA was characterized by its sample score, which shows strength of the signal. In the present report, the translation initiation signal sequences of rat preproinsulin and its mutant mRNAs were analyzed using a quantification method proposed previously. A consensus sequence for the signal, (GCC)GCC(A or G)CC ATG G, has been proposed, but actual initiation sequences differ from it to a greater or lesser degree. Concerning the translation initiation signals in vertebrate mRNAs, both the ATG initiation codon and the sequences flanking the initiation codon are required to direct the position of initiation. ![]()
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