 
Function: In this model, SmpB protein binds tmRNA and the complex is aminoacylated by Ala-RS (a). The aminoacylated-tmRNA-SmpB complex is recognized by EF-Tu-GTP (b) to form a stalled ribosome recognition complex. Ribosomes stalled at the 3’-end of nonstop mRNAs are initially recognized by this quaternary-complex in a pre-accommodated state (c). Proper accommodation of the tRNA-like domain of tmRNA into the ribosomal A-site is then triggered by contacts, perhaps elicited by the C-terminal tail of SmpB, that promote GTP hydrolysis on EF-Tu. Accommodation is followed by the first transpeptidation reaction that links the alanine charge of tmRNA to the incomplete polypeptide (d). tmRNA then switches from a tRNA-like mode to an mRNA-like mode, engaging the ribosome on its own peptide reading frame (e), concomitant with the displacement of the defective mRNA (f). The damaged mRNA is selectively recognized and degraded by RNase R, in an SmpB and tmRNA dependent manner. The rescued ribosome resumes translation with the tmRNA ORF as its surrogate template (g). Translation terminates on the tmRNA-encoded stop codon, permitting recycling of stalled ribosomes into the cellular pool (h). The nascent polypeptide, now marked with an 11-amino acid degradation tag, is released from the translation machinery (i), and specifically recognized and degraded by C-terminal specific cellular proteases (j).
|
The trans-Translation Model for tmRNA
|
