I: A ribosome (gray) at the 3'-end of broken messenger RNA (blue) is stuck with
the polypeptide chain (gold) incompleted. tRNA (pigeon blue) for the last functional codon remains bound to the P-site (yellow).
II: tmRNA (pigeon blue and purple), charged with alanine (red ball), enters the A-site (red).
III: Translation resumes on the tag-peptide coding region (purple) of tmRNA.
IV: The ribosome reaches tmRNA stop codon, releases a tagged (green balls) polypeptide, and joins the pool of active ribosomes. The tagged polypeptide will be digested by bacterial proteases.
See article by Keiler K.C., Waller P.R., and Sauer R.T. (1996) Science,271:990-993. [Abstract] and Wikipedia.