Info

stalled ribosome 3'end-broken mRNA

EF-Tu-GDP

transpeptidation translocation and replacement of rnRN A

SDfl

EF-Tu-GDP

SsrA RNA

recognition by SsrA RNA

continued translocation of rnRNA reading frame transpeptidation translocation and replacement of rnRN A

continued translocation of rnRNA reading frame

degradation by cellular proteases

SDfl

EF-Tu-GDP

stalled ribosome 3'end-broken mRNA

EF-Tu-GDP

SsrA RNA

recognition by SsrA RNA

FIGURE 14-39 The tmRNA and SsrA rescue ribosomes stalled on prematurely terminated mRNAs. The SsrA RNA mimics a tRNA but can only bind a ribosomo that is stalled at tine 3' end of an rnRNA. Once bound, the SsrA mRNA substitutes part of its sequence to act as a new "mRNA"

Thus, translation products arising from broken mRNAs are rapidly cleared to prevent these defective proteins from harming the cell.

How does the .SsrA RNA bind to only stalled ribosomes? Because of the large size of SsrA [it is more than four times bigger than a standard tRNA), it cannot bind to the A site during normal elongation. In contrast, when the 3' end of the mRNA is missing, additional room is created in the A site to accommodate the larger RNA. Thus, only ribosomes stalled at the 3' end of an mKNA represent a potential binding site for the SsrA RNA.

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