Transfer-messenger RNA (abbreviated tmRNA, also known as 10Sa
RNA and by its genetic name SsrA) is a bacterial RNA molecule with
dual tRNA-like and messenger RNA-like properties. The tmRNA forms a ribonucleoprotein
complex (tmRNP) together with Small Protein B (SmpB), Elongation Factor
Tu (EF-Tu), and ribosomal protein S1. In trans-translation, tmRNA and
its associated proteins bind to bacterial ribosomes which have stalled in the
middle of protein biosynthesis, for example when reaching the end of a messenger
RNA which has lost its stop codon. The tmRNA is remarkably versatile: it
recycles the stalled ribosome, adds a proteolysis-inducing tag to the
unfinished polypeptide, and facilitates the degradation of the aberrant messenger
RNA. In the majority of bacteria these functions are carried out by standard one-piece
tmRNAs. In other bacterial species, a permuted ssrA gene produces a two-piece
tmRNA in which two separate RNA chains are joined by base-pairing.
E. coli contains a small RNA, encoded by the ssrA gene, is synthesized as a 457 nt precursor RNA that is processed by RNaseE to a mature 363 nt RNA. This RNA is also known as tmRNA or 10Sa RNA.
The ssrA RNA has a number of important properties:
When a ribosome stalls, the ssrA
RNA charged with alanine is brought to the A-site of the ribosome
by the SsrB protein. Peptidyl
transferase activity transfers the nascent polypeptide to the
alanine attached to ssrA.
The mRNA template is also displaced by the ssrA RNA. Further protein synthesis now uses ssrA as a template and ten further amino acids (ANDENYALAA) are added to the C-terminal end of the polypeptide.
However, the final two amino acids that are added (AA) mark the new protein for proteolysis by the two proteases ClpAP and ClpXP.
Thus any proteins that are only partially synthesized by stalled ribosomes can be rapidly destroyed and turned over.
further reading...
E. coli contains a small RNA, encoded by the ssrA gene, is synthesized as a 457 nt precursor RNA that is processed by RNaseE to a mature 363 nt RNA. This RNA is also known as tmRNA or 10Sa RNA.
The ssrA RNA has a number of important properties:
- Its secondary and tertiary structure partially resembles
that of tRNA
- It can be charged with alanine
- It can be used as an mRNA which codes for a 10 amino acid
long oligopeptide: ANDENYALAA.
The mRNA template is also displaced by the ssrA RNA. Further protein synthesis now uses ssrA as a template and ten further amino acids (ANDENYALAA) are added to the C-terminal end of the polypeptide.
However, the final two amino acids that are added (AA) mark the new protein for proteolysis by the two proteases ClpAP and ClpXP.
Thus any proteins that are only partially synthesized by stalled ribosomes can be rapidly destroyed and turned over.
further reading...
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