Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 189

As mentioned above, a few mRNAs in bacteria do not have standard TIRs with leader regions containing S-D sequences. In these rare mRNAs, the initiator codon can be right at or very close to the 5′ end of the mRNA. How the ribosome recognizes such an initiator codon and initiates translation is not fully understood, but the mechanism seems to be very different from that of initiation at a normal TIR. There is some evidence that a complex first forms between fMet-tRNA^fMet, IF2, and the small subunit of the ribosome. This complex may then help recognize the initiation codon in the absence of upstream sequences to help distinguish the initiation codon. Other evidence suggests that the 70S ribosome itself recognizes the leaderless initiator codon.