06-15-2014, 07:50 PM
PiRNA – piwi-interacting RNA
PiRNAs form the largest class of small non-coding RNA molecules and they got their name because of the complexes they form with piwi proteins (RNA-protein complexes). They are typically a bit longer (about 26-31 nucleotides) than the regular miRNA or siRNA (about 21 nucleotides). They are found in germ line cells, and are mostly responsible for silencing of the genes both on genomic level (epigenetic silencing) and on transcriptional level (post-transcriptional gene silencing).
It has been hard to establish all of their functions so far, because there is great variation in both piRNA sequences and in piwi proteins. Until now, more than 50,000 unique piRNA sequences have been identified only in mice. However, the theory about gene silencing is pretty strong. Scientists think that piRNA are mostly responsible for the silencing of transposons as most of them are antisense to transposon sequences. They are expressed mostly during the development of the embryo, but they are also necessary for spermatogenesis.
The way they perform silencing of a particular DNA sequence is through RISC (RNA-induced silencing complex). They form complex with piwi proteins and they direct them to the transposons. It has been noticed that the lack of PIWI proteins results in an increased expression of transposons.
While some characteristics are conserved for the class of piRNA molecules (like the fact that they do not have secondary structure motifs), some are found in only specific piRNAs belonging to some organisms. For example, some piRNAs have a 5’ monophosphate and a 3’ modification which blocks 2’ and 3’ oxygen, which increases the stability of the molecules. These modifications have been confirmed in organisms like C. elegans, D. melanogaster, zebrafish, mice and rats.
PiRNAs are usually grouped together in clusters and can be found throughout the genome in numbers ranging from ten to several thousands of them. This results in some large sequences of up to a hundred kb. They are found both in mammals and in invertebrates and both in male and female germlines.
We actually know little about piRNAs considering how many different types of them are there, and we have yet to use the newest technologies in order to find out more about them.
PiRNAs form the largest class of small non-coding RNA molecules and they got their name because of the complexes they form with piwi proteins (RNA-protein complexes). They are typically a bit longer (about 26-31 nucleotides) than the regular miRNA or siRNA (about 21 nucleotides). They are found in germ line cells, and are mostly responsible for silencing of the genes both on genomic level (epigenetic silencing) and on transcriptional level (post-transcriptional gene silencing).
It has been hard to establish all of their functions so far, because there is great variation in both piRNA sequences and in piwi proteins. Until now, more than 50,000 unique piRNA sequences have been identified only in mice. However, the theory about gene silencing is pretty strong. Scientists think that piRNA are mostly responsible for the silencing of transposons as most of them are antisense to transposon sequences. They are expressed mostly during the development of the embryo, but they are also necessary for spermatogenesis.
The way they perform silencing of a particular DNA sequence is through RISC (RNA-induced silencing complex). They form complex with piwi proteins and they direct them to the transposons. It has been noticed that the lack of PIWI proteins results in an increased expression of transposons.
While some characteristics are conserved for the class of piRNA molecules (like the fact that they do not have secondary structure motifs), some are found in only specific piRNAs belonging to some organisms. For example, some piRNAs have a 5’ monophosphate and a 3’ modification which blocks 2’ and 3’ oxygen, which increases the stability of the molecules. These modifications have been confirmed in organisms like C. elegans, D. melanogaster, zebrafish, mice and rats.
PiRNAs are usually grouped together in clusters and can be found throughout the genome in numbers ranging from ten to several thousands of them. This results in some large sequences of up to a hundred kb. They are found both in mammals and in invertebrates and both in male and female germlines.
We actually know little about piRNAs considering how many different types of them are there, and we have yet to use the newest technologies in order to find out more about them.
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