In bacteria (specifically E. coli), six components are involved in translation initiation: (1) mRNA, (2) small ribosomal subunit, (3) the large ribosomal subunit, (4) the initiator tRNA, (5) three essential initiation factor proteins, and (6) GTP.
The preinitiation complex forms when the authentic start codon sequence is identified by base pairing that occurs between the 16S rRNA in the 30S ribosome and a short mRNA sequence located a few nucleotides upstream of the start codon in the 5? UTR of mRNA. The complex knows where to assemble on the mRNA because of the
Shine-Dalgarno sequence, a purine-rich sequence of about six nucleotides located three to nine nucleotides upstream of the start codon. A complementary pyrimidine-rich segment containing the sequence UCCUCC is found near the 3? end of 16S rRNA, and it pairs with the Shine-Dalgarno sequence to position the mRNA on the 30S subunit such that it recognizes the correct start codon.
In eukaryotes, the preinitiation complex is recruited to the 5?-cap region of mRNA, located at the end of the 5? UTR. Once the eukaryotic initiation complex is formed, it embarks on a process called scanning, in which it uses ATP hydrolysis to move the small ribosomal subunit through the 5? UTR in search of the start codon. About 90% of eukaryotic mRNAs use the first AUG encountered by the initiation complex as the start codon. The
initiation complex is able to accurately locate the authentic start codon because the codon is embedded in a consensus sequence, the Kozak sequence, which reads 5' - ACCAUGG - 3'.
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