R

BOX 14-4 FIGURE 1 Puromycin terminates translation by minuting a tRNA in the A site. Puromycin binds in the A site and participates in peptide bond formation. Once completed, puromycin and any associated polypeptide diffuses out of the ribosome.

reading frame? Such an mRNA r,an be generated by incomplete transcription or nuclease action. Translation of this type of mRNA can initiate normally and continué until tile 3' end of the mRNA is reached. At this point, the ribosome cannot proceed. There is no codon either to bind an ami noacy)-tRNA or to bind a release factor. Without some mechanism to release them from these defective mRNAs, many ribo-somos would be permanently trapped, removing them from polypeptide synthesis. In prokaryotic cells, such stalled ribosomes are rescued by the action of a chimeric RNA molecule that is part tRNA and part mRNA, called a tmRNA.

SsrA is a 457-nuclcotide tmRNA that includes a region at its 3' end that strongly resembles tRNAAlu (Figure 14-39), This similarity allows the SsrA RNA to be charged with alanine and to bind EF-Tu-GTP. When a ribosome is stalled at the 3' end of an mRNA, the SsrAAla-EF-Tu-GTP complex binds to the A site of the ribosome and participates in the pep-tidyl lransfera.se reaction, as would any other tRNA. Translocation of the peptidy-SsrA RNA results in the release of the broken mRNA. Remarkably, translocation of the SsrA RNA also results in a portion of this RNA entering the mRNA-binding channel of the ribosome. This portion of the SsrA RNA extends the upen-reading frame of the incomplete mRNA by ten codons followed by a stop codon. The net result of SsrA binding is that when the defective mRNA is released from the ribosome, the incomplete polypeptide is fused to a ten-amino-acid "tag" at its carboxyl terminus and the ribosome is recycled. Interestingly, the ten-amino-acid tag is recognized by cellular proteases that rapidly degrade the tag and the truncated polypeptide to which it is attached.

pepfidyi-tRNA in the P site

P site

N A site HjC CHS

puromycin in the A site peptidyl c_.0 center NH OH

pepfidyi-tRNA in the P site

peptidyl c_.0 center NH OH

P site

N A site HjC CHS

puromycin in the A site

Translation -Dependent Regulation af mRNA and Protein Stability

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