Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 197

A problem occurs when the ribosome reaches the 3′ end of an mRNA without encountering a termination codon. This might happen fairly often, because mRNA is constantly being cleaved or degraded (often from the 3′ end), and transcription may terminate prematurely, resulting in a truncated mRNA. The release factors can function only at a termination codon, so the ribosome will stall on a truncated mRNA. Not only would this cause a traffic jam and sequester ribosomes in a nonfunctional state, but also the protein that is made will be defective because it is shorter than normal, and accumulation of defective proteins may cause problems for the cell. This is where a small RNA called transfer-messenger RNA (tmRNA) comes to the rescue. As the name implies, tmRNA is both a tRNA and an mRNA, as shown in Figure 2.32. It can be aminoacylated with alanine by alanyl-tRNA synthetase like an alanyl-tRNA, and it also contains a short open reading frame (ORF) that terminates in a termination codon like an mRNA. If the ribosome reaches the end of an mRNA without encountering a termination codon, tm-RNA (together with an accessory protein) enters the A site of the stalled ribosome, and alanine is inserted as the next amino acid of the polypeptide. Then, by a process that is not well understood, the ribosome shifts from translating the ORF on the mRNA to translating the ORF on the tmRNA, where it soon encounters the termination codon at the end of the ORF. The release factors then release the ribosome and the truncated polypeptide, which now contains a short additional “tag” sequence of about 10 amino acids encoded by the tmRNA. The tag sequence that has been attached to the carboxy end of the truncated polypeptide is recognized by the ClpXP protease (see below), which degrades the entire defective polypeptide. In some cases, tmRNA-mediated degradation may play a regulatory role, allowing the targeted degradation of proteins until they are needed (see Keiler and Feaga, Suggested Reading).