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Titel    microRNA
Verlag    (Wikipedia)
Jahr    2007
URL    http://en.wikipedia.org/w/index.php?title=MicroRNA&oldid=157563157

Literaturverz.   

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Fußnoten    nein
Fragmente    4


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[1.] Ww/Fragment 002 17 - Diskussion
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In genetics, miRNAs are single-stranded RNA molecules of about 21-23 nucleotides in length, which regulate gene expression. miRNAs are encoded by genes that are transcribed from DNA but not translated into protein (non-coding RNA); instead they are processed from primary transcripts known as pri-miRNAs to short stem-loop structures called pre-miRNAs and finally to function miRNAs. Mature miRNA molecules are partially complementary to one or more messenger RNA (mRNA) molecules, and their main function is to down-regulate gene expression.

1.1.3.1 Formation and processing of miRNA

The genes encoding miRNAs are much longer than the processed mature miRNA molecule; miRNAs are first transcribed as primary transcripts or pri-miRNAs with a cap and poly-A tail and processed to short, 70-nucleotide stem-loop structures known as pre-miRNAs in the cell nucleus. This [processing is performed in animals by a protein complex known as the Microprocessor complex, consisting of the nuclease Drosha and the and the double-stranded RNA binding protein Pasha (Denli et al, 2004).]

In genetics, microRNAs (miRNA) are single-stranded RNA molecules of about 21-23 nucleotides in length regulating gene expression. miRNAs are encoded by genes that are transcribed from DNA but not translated into protein (non-coding RNA); instead they are processed from primary transcripts known as pri-miRNA to short stem-loop structures called pre-miRNA and finally to functional miRNA. Mature miRNA molecules are partially complementary to one or more messenger RNA (mRNA) molecules, and their main function is to downregulate gene expression. [...]

[...]

The genes encoding miRNAs are much longer than the processed mature miRNA molecule; miRNAs are first transcribed as primary transcripts or pri-miRNA with a cap and poly-A tail and processed to short, 70-nucleotide stem-loop structures known as pre-miRNA in the cell nucleus. This processing is performed in animals by a protein complex known as the Microprocessor complex, consisting of the nuclease Drosha and the double-stranded RNA binding protein Pasha.[3]


3. Denli AM, Tops BB, Plasterk RH, Ketting RF, Hannon GJ. (2004). Nature 432(7014):231-5.

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[2.] Ww/Fragment 003 01 - Diskussion
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[This] processing is performed in animals by a protein complex known as the Microprocessor complex, consisting of the nuclease Drosha and the double-stranded RNA binding protein Pasha (Denli et al, 2004). These pre-miRNAs are then processed to mature miRNAs in the cytoplasm by interaction with the endonuclease Dicer, which also initiates the formation of the RNA-induced silencing complex (RISC) (Bernstein et al, 2001). This complex is responsible for the gene silencing observed due to miRNA expression and RNA interference. The pathway in plants varies slightly due to the lack of Drosha homologs; instead, Dicer homologs alone affect several processing steps (Kurihara and Watanabe, 2004).

It has been shown that the efficient processing of pre-miRNAs by Drosha requires the presence of extended single-stranded RNAs on both 3’- and 5’-ends of hairpin molecules (Zeng et al, 2005). This study showed that these motifs could be of different composition while their defined length is of high importance for processing to take place. Findings were confirmed in another work by Han et al (2004). Using bioinformatics tools the folding of 321 human and 68 fly pri-miRNAs was analysed. 280 human and 55 fly pri-miRNAs were selected for further study excluding those molecules where folding showed the presence of multiple loops. All human and fly pri-miRNAs contained very similar structural regions, which the authors called ‘’basal segments’’, ‘’lower stem’’, ’’upper stem’’ and ‘’terminal loop’’. Based on the encoding position of miRNAs, in the 5’-strand (5’-donors) or 3’-strand (3’-donors), thermodynamic profiles of pri-miRNAs were determined (Zeng et al, 2005). Subsequent experiments showed that Drosha complex cleaves RNA molecules ~2 helical turns away from the terminal loop and ~1 turn away from basal segments. In most analyzed molecules this region contains unpaired nucleotides and the free energy of the duplex is relatively high compared to lower and upper stem regions (Zeng and Cullen, 2005).

Most pre-miRNAs don’t have a perfect double-stranded RNA (dsRNA) structure topped by a terminal loop. There are few possible explanations for [such selectivity.]

This processing is performed in animals by a protein complex known as the Microprocessor complex, consisting of the nuclease Drosha and the double-stranded RNA binding protein Pasha.[3] These pre-miRNAs are then processed to mature miRNAs in the cytoplasm by interaction with the endonuclease Dicer, which also initiates the formation of the RNA-induced silencing complex (RISC).[4] This complex is responsible for the gene silencing observed due to miRNA expression and RNA interference. The pathway in plants varies slightly due to their lack of Drosha homologs; instead, Dicer homologs alone effect several processing steps.[5]

Zeng et al. have shown that efficient processing of pre-miRNA by Drosha requires presence of extended single-stranded RNA on both 3'- and 5'-ends of hairpin molecule. They demonstrated that these motifs could be of different composition while their length is of high importance if processing is to take place at all. Their findings were confirmed in another work by Han et al. Using bioinformatical tools Han et al. analysed folding of 321 human and 68 fly pri-miRNAs. 280 human and 55 fly pri-miRNAs were selected for further study, excluding those molecules which folding showed presence of multiple loops. All human and fly pri-miRNA contained very similar structural regions, which authors called 'basal segments', 'lower stem', 'upper stem' and 'terminal loop'. Based on the encoding position of miRNA, i.e. in the 5'-strand (5'-donors) or 3'-strand (3'-donors), thermodynamical profiles of pri-miRNA were determined. Following experiments have shown that Drosha complex cleaves RNA molecule ~2 helical turns away from the terminal loop and ~1 turn away from basal segments. In most analysed molecules this region contains unpaired nucleotides and the free energy of the duplex is relatively high compared to lower and upper stem regions.

Most pre-miRNAs don't have a perfect double-stranded RNA (dsRNA) structure topped by a terminal loop. There are few possible explanations for such selectivity. One could be that dsRNAs longer than 21 base pairs activate interferon response and anti-viral machinery in the cell.


3. Denli AM, Tops BB, Plasterk RH, Ketting RF, Hannon GJ. (2004). Nature 432(7014):231-5.

4. Bernstein E, Caudy AA, Hammond SM, Hannon GJ. (2001). Role for a bidentate ribonuclease in the initiation step of RNA interference. Nature 409(6818):363-6.

5. Kurihara Y, Watanabe Y. (2004). Arabidopsis micro-RNA biogenesis through Dicer-like 1 protein functions. Proc Natl Acad Sci USA 101(34):12753-8.

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[There are few possible explanations for] such selectivity. One could be that dsRNAs longer than 21 base pairs activate an interferon response and the anti-viral machinery in the cell. Another plausible explanation could be that the thermodynamical profile of

pre-miRNAs determines which strand will be incorporated into the Dicer complex. Indeed, the aforementioned study by Han et al. demonstrated very clear similarities between pri-miRNAs encoded in respective (5’- or 3’-) strands.

When Dicer cleaves the pre-miRNA stem-loop, two complementary short RNA molecules are formed, but only one becomes integrated into the RISC complex. This strand is known as the guide strand and is selected by the argonaute protein, the catalytically active RNase in the RISC complex, on the basis of the stability of the 5’ end (Preall et al, 2006). The remaining strand, known as the anti-guide or passenger strand is degraded as a RISC complex substrate (Gregory et al, 2005). After integration into the active RISC complex, miRNAs base pair with their complementary mRNA molecules and induce mRNA degradation by argonaute proteins. It is as yet unclear how the activated RISC complex locates the mRNA targets in the cell, though it has been shown that the process is not coupled to ongoing protein translation from the mRNA (Sen et al, 2005).

1.1.3.2 Cellular functions of miRNA

The miRNAs appear to be important for gene regulation. An individual miRNA is complementary to a part of one or more messenger RNAs (mRNAs). Animal miRNAs are usually complementary to a site in the 3’ UTR whereas plant miRNAs are usually complementary to coding regions of mRNAs. The annealing of the miRNA to the mRNA then inhibits protein translation, but sometimes facilitates cleavage of the mRNA. This is thought to be the primary mode of action of plant miRNAs. In such cases, the formation of the double-stranded RNA through the binding of the miRNA triggers the degradation of the mRNA transcript through a process similar to RNAi, though [in other cases it is believed that the miRNA complex blocks the protein translation machinery or otherwise prevents protein translation without causing the mRNA to be degraded.]

There are few possible explanations for such selectivity. One could be that dsRNAs longer than 21 base pairs activate interferon response and anti-viral machinery in the cell. Another plausible explanation could be that thermodynamical profile of pre-miRNA determines which strand will be incorporated into Dicer complex. Indeed, aforementioned study by Han et al. demonstrated very clear similarities between pri-miRNAs encoded in respective (5'- or 3'-) strands.

When Dicer cleaves the pre-miRNA stem-loop, two complementary short RNA molecules are formed, but only one is integrated into the RISC complex. This strand is known as the guide strand and is selected by the argonaute protein, the catalytically active RNase in the RISC complex, on the basis of the stability of the 5' end.[6] The remaining strand, known as the anti-guide or passenger strand, is degraded as a RISC complex substrate.[7] After integration into the active RISC complex, miRNAs base pair with their complementary mRNA molecules and induce mRNA degradation by argonaute proteins, the catalytically active members of the RISC complex. It is as yet unclear how the activated RISC complex locates the mRNA targets in the cell, though it has been shown that the process is not coupled to ongoing protein translation from the mRNA.[8]

Cellular functions

The function of miRNAs appears to be in gene regulation. For that purpose, a miRNA is complementary to a part of one or more messenger RNAs (mRNAs). Animal miRNAs are usually complementary to a site in the 3' UTR whereas plant miRNAs are usually complementary to coding regions of mRNAs. The annealing of the miRNA to the mRNA then inhibits protein translation, but sometimes facilitates cleavage of the mRNA. This is thought to be the primary mode of action of plant miRNAs. In such cases, the formation of the double-stranded RNA through the binding of the miRNA triggers the degradation of the mRNA transcript through a process similar to RNA interference (RNAi), though in other cases it is believed that the miRNA complex blocks the protein translation machinery or otherwise prevents protein translation without causing the mRNA to be degraded.


6. Preall JB, He Z, Gorra JM, Sontheimer EJ. (2006). Short interfering RNA strand selection is independent of dsRNA processing polarity during RNAi in Drosophila. Curr Biol 16(5):530-5.

7. Gregory RI, Chendrimada TP, Cooch N, Shiekhattar R. (2005). Human RISC couples microRNA biogenesis and posttranscriptional gene silencing. Cell 123(4):631-40.

8. Sen GL, Wehrman TS, Blau HM. (2005). mRNA translation is not a prerequisite for small interfering RNA-mediated mRNAs cleavage. Differentiation 73(6):287-93.

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[In such cases, the formation of the double-stranded RNA through the binding of the miRNA triggers the degradation of the mRNA transcript through a process similar to RNAi, though] in other cases it is believed that the miRNA complex blocks the protein translation machinery or otherwise prevents protein translation without causing the mRNA to be degraded. miRNAs may also target methylation of genomic sites which correspond to targeted mRNAs. miRNAs function in association with a complement of proteins collectively termed the miRNP (micro-ribonucleic protein).

This effect was first described for the worm C. elegans in 1993 (Lee et al., 1993). As of 2002, miRNAs have been confirmed in various plants and animals, including C. elegans, human and the plant Arabidopsis thaliana. Genes have been found in bacteria that are similar in the sense that they control mRNA abundance or translation by binding a mRNA by base pairing, however they are not generally considered to be miRNAs because the Dicer enzyme is not involved.

In plants, similar RNA species termed short-interfering RNAs are used to prevent the transcription of viral RNA. While this siRNA is double-stranded, the mechanism seems to be closely related to that of miRNAs, especially taking the hairpin structures into account. siRNAs are also used to regulate cellular genes, as miRNAs do.

In such cases, the formation of the double-stranded RNA through the binding of the miRNA triggers the degradation of the mRNA transcript through a process similar to RNA interference (RNAi), though in other cases it is believed that the miRNA complex blocks the protein translation machinery or otherwise prevents protein translation without causing the mRNA to be degraded. miRNAs may also target methylation of genomic sites which correspond to targeted mRNAs. miRNAs function in association with a complement of proteins collectively termed the miRNP.

This effect was first described for the worm C. elegans in 1993 by Victor Ambros and coworkers (Lee et al., 1993). As of 2002, miRNAs have been confirmed in various plants and animals, including C. elegans, human and the plant Arabidopsis thaliana. Genes have been found in bacteria that are similar in the sense that they control mRNA abundance or translation by binding an mRNA by base pairing, however they are not generally considered to be miRNAs because the Dicer enzyme is not involved.

In plants, similar RNA species termed short-interfering RNAs siRNAs are used to prevent the transcription of viral RNA. While this siRNA is double-stranded, the mechanism seems to be closely related to that of miRNA, especially taking the hairpin structures into account. siRNAs are also used to regulate cellular genes, as miRNAs do.

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