Stalled ribosomes, rescue

Truncated mRNAs lacking a stop codon cause the synthesis of incomplete peptide chains and stall translating ribosomes. In bacteria, a ribonucleoprotein complex composed of tmRNA (transfer-messenger RNA), a molecule that combines the functions of tRNA and mRNA, and small protein B (SmpB) rescues stalled ribosomes (Haebel et al., 2004). The dual function of this molecule as both tRNA and mRNA facilitates a trans-translation reaction in which a ribosome can switch between translation of a truncated mRNA and the tag sequence of tmRNA. 

Figure 29-16 Schematic diagram of the tmRNA structure and its function in the rescue of ribosomes stalled at the end of a messenger RNA that has been broken and has lost its inframe termination codon. After it binds into the ribosomal A site the tmRNA, which has been charged with alanine, undergoes the peptidyltransferase reaction and translocation to the P site. Then it lays down its mRNA-like coding sequence, which is used by the ribosome to add ten more amino acids to form the 11-residue C-terminal degradation signal A ANDENYALAA. This induces rapid degradation of the imperfect protein that has been formed.4363...

R 42 P.W. Haebel, S. Gutmann and N. Ban, Dial tm for Rescue tmRNA Engages Ribosomes Stalled on Defective mRNAs , p. 58... 

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