Transfer-messenger RNA

TmRNA is the short form for transfer -messenger RNA. Is a small, stable RNA (short RNA). The name is a combination of tRNA ( transfer RNA ) and mRNA (messenger RNA ). The name derives from this is the ability to combine properties of both types of RNA. Other common names are obsolete but 10S RNA and RNA 10Sa.

Occurrence

Genes for tmRNA were found in each bacterial genome, including without bacteria with a reduced amount of genetic material. In contrast, tmRNA was not in archaea yet - identified in eukaryotes - with the exception of some organelles.

Gene

The gene encoding the tmRNA, is ssrA. It is like other tRNAs synthesized from a Vorläufertranskript and processed in a similar manner by ribonucleases further, for example by RNase P or RNase E. bacteria adopting the CCA sequence at the 5 ' end later ( post-transcriptionally ), also do this for their pre- tmRNAs. This happens, for example, in Bacillus subtilis.

The essential for the function of tmRNA SmpB protein encoded by the gene SMPB.

In most bacterial species that tmRNA is not essential, but can lead to growth defects, for example, by stress responses. For Neisseria gonorrhoeae, Shigella flexneri, Mycoplasma genitalium or Haemophilus influenzae, however, the activity of tmRNA is essential for survival.

Construction

The tRNA portion of tmRNA has both a TΨC strain and an acceptor stem, which ends with the typical tRNA sequence CCA - 5 '. However, it lacks the anticodon stem and the D- loop. A specific open reading frame (ORF) and is similar to a mRNA coding for a protein sequence that is attached later to an existing protein (see also function). In tmRNA also four three to four pseudoknots were identified, their exact function is still unclear.

The tRNA structural elements of tmRNA correspond to those of a tRNA that recognizes alanine ( tRNAAla ). So is the acceptor before the special G • C wobble base pair, which is essential for recognition by the alanine -tRNA synthetase. Consequently, the tmRNA is loaded by these alanine -tRNA synthetase with alanine.

For the function of a normal tRNA anticodon stem of is essential. In the tmRNA will be replaced by a special protein: SmpB. It must bind to the tmRNA so that it can fulfill its function. A structure of the tRNA -like domain of the tmRNA is identified in a complex with the SMPB protein.

Operation

Their shape is the tmRNA as a tRNA able to dock with a waiting ribosome and so the translation process to resume. Here, the mRNA part of tmRNA own is used as a new reading frame. The process of translation is now continued with this what you referred to as trans - Translation. Thus, the peptide chain so far produced is extended by a specific mark in the form of a peptide chain residue. This residue serves as a signal for subsequent degradation. A stop codon on the mRNA part causes at the end of the release of the ribosome.

Functions

Quality control

In the translation of mRNA on ribosomes to protein, it is possible that the mRNA lacking a stop codon. This happens, for example, because the transcription of the mRNA was aborted before reaching the stop codon. Also, a frame shift can cause in-frame stop codon occurs no more. Alternately, you may read during translation, which occurs in the mRNA stop codon. In all these examples, the ribosome finally arrives to the 3 ' end of the mRNA and remains in this state; it can not "back" in the 5'- or "forward" in the 3 ' direction.

Since this complex of ribosome and mRNA is stable in the cellular environment, these ribosomes are removed from the translation pool and linger useless in the cytosol. Such an erroneous translation is done in Escherichia coli per cell division up to 13,000 times, on average, each ribosome in the cell is affected during a cell cycle it.

In addition, the produced thereby and possibly released proteins and peptides fulfill any task because they have not reached their full length. In addition, they may be potentially damaging to the cell.

To counter this, the tmRNA has the following important functions:

Regulation of gene expression

Besides task in quality control for trans - translation plays an important role in the regulation of gene expression of some substrates.

E. coli trans - translation is used for the regulation of LACI. LacI is the homotetrameric repressor of the lac operon, which - if there is sufficient available glucose - binds to the promoter sites of the lac operon. For a reading of the genes lacZ, lacY and lacA is prevented which would be necessary for the degradation of lactose. The LacI repressor autoinhibiert its own synthesis; it binds to two lac operator sites, O1 and O3. O1 lies with the lacZYA promoter, O3 at the end of the coding region for lacI itself through binding of LacI to O1 and O3 occurs for the intervening DNA to form a loop. Now, when an RNA polymerase reads the lacI gene ( transcription), it passes prior to reaching the stop codon of this loop and falls off. The transcribed LacI is so incomplete, among other things, it lacks the stop codon.

Ribosomes that read this incomplete LacI mRNA, would eventually be stuck at the 3 ' end. Is ensured only through the process of tmRNA in trans - translation, that these ribosomes freed and the incomplete LacI ( LacI *) is supplied to the degradation. The latter is the crucial role in the regulation of the trans - translation, because it has been shown that LacI * is still an active repressor; the regulation of gene expression would thus fail without the process of trans - translation.

Another example of the role of trans - translation is proposed as part of a stress response. Under stress conditions, the toxin RelE for a global, cellular translational stop ensures by mRNAs are cut. This is to ensure that valuable resources are diverted immediately for essential cellular processes - the translation is a costly operation. If the stress-inducing cause has been overcome, RelE is inactivated. Through the process of trans - translation, the ribosomes are freed, so that the cell can operate normally again.