Evolution of the ssrA degradation tag in Mycoplasma: Specificity switch to a different protease

Eyal Gur, Robert T. Sauer

Research output: Contribution to journalArticlepeer-review

46 Scopus citations


Stalled ribosomes in bacteria are rescued by the tmRNA system. In this process, the nascent polypeptide is modified by the addition of a short C-terminal sequence called the ssrA tag, which is encoded by tmRNA and allows normal termination and release of ribosomal subunits. In most bacteria, ssrA-tagged proteins are degraded by the AAA+ protease, ClpXP. However, in bacterial species of the genus Mycoplasma, genes for ClpXP and many other proteins were lost through reductive evolution. Interestingly, Mycoplasma ssrA tag sequences are very different from the tags in other bacteria. We report that ssrA-tagged proteins in Mesoplasma florum, a Mycoplasma species, are efficiently recognized and degraded by the AAA+ Lon protease. Thus, retaining degradation of ssrA-tagged translation products was apparently important enough during speciation of Mycoplasma to drive adaptation of the ssrA tag to a different protease. These results emphasize the importance of coupling proteolysis with tmRNA-mediated tagging and ribosome rescue.

Original languageEnglish
Pages (from-to)16113-16118
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number42
StatePublished - 21 Oct 2008
Externally publishedYes


  • AAA+ protease
  • ClpX
  • Lon
  • Reductive evolution

ASJC Scopus subject areas

  • General


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