snoRNA

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snoRNA ( English small nucleolar ribonucleic acid , small nucleolar RNA ') are RNA in eukaryotes and some archaea involved in the processing of others and modification of ribonucleic acids - in particular ribosomal RNA ( rRNA are involved and have a role in -) genomic imprinting play.

properties

SnoRNA do not code for proteins , but work as a guide RNA by bringing the enzymes to the right places on the RNA. In the cell , snoRNAs - like almost all other RNAs - are not present naked , but associated with proteins as ribonucleoprotein , which is why one speaks of snoRNPs ( English small nucleolar ribonucleoprotein particle ).

The modifications introduced into the ribosomal RNA by these snoRNPs are essential for the function of the ribosome in the course of translation , and those in the snRNAs are essential for splicing . As the name implies, the snoRNPs are mostly found in the nucleoli , where they modify the rRNAs, with the exception of the related scaRNAs (see below). In eukaryotes, SnoRNA are transported into the cell nucleus through Crm1 .

In addition to the modifications of nucleic acids supported by the guide RNAs, there is also a direct modification without the help of this small RNA (e.g. DNA methylation by DNA methyltransferases, methylation of the 5'-cap structure or RNA editing, e.g. . by ADAR enzymes ).

snoRNA families

In general, one can differentiate between two types of snoRNAs, the box C / D and the box H / ACA type, each of which is responsible for a different modification of the RNA. The name comes from different sequence and folding motifs in the respective RNAs.

In addition, both types of RNAs are associated with different proteins that are involved in the correct recognition of the target and carry out the actual modification reaction. These are the following proteins (in Saccharomyces cerevisiae ):

  • box C / D: Fibrillarin / Nop1, Nop56, Nop58, snu13
  • box H / ACA: Nap57, Nop10, Nhp2, Gar1 / Dyskerin

Box C / D RNAs are involved in the 2'O- methylation of riboses . They recognize the target RNA based on complementary sequences and bind them to the right place. The catalytic subunit, the protein fibrillarin, then becomes active and transfers the methyl group from S- adenosylmethionine to the ribose.

Box H / ACA snoRNPs work in the same way, here too the snoRNA recognizes the target RNA, but the subsequent reaction is not methylation, but a conversion of uridine to pseudouridine by the protein dyskerin.

scaRNAs

Only recently was another type of guide modification RNA discovered, the so-called scaRNAs ( small cajal body localized RNAs ). These RNAs have some similarities with snoRNAs, but contain both box C / D and H / ACA motifs, which is why they are also referred to as chimeric RNAs. In contrast to the snoRNAs, the scaRNAs can be found in the Cajal Bodies (also known as Coiled Bodies , CBs) (nuclear bodies discovered about 100 years ago by Ramon y Cajal , probably used in recycling and the biogenesis of spliceosomal snRNA ). They also do not catalyze the modification of ribosomal RNAs, but rather that of snRNAs and are therefore essential for splicing .

Web links

Individual evidence

  1. ↑ KR Phipps, J. Charette, SJ Baserga: The small subunit processome in ribosome biogenesis? Progress and prospects. In: Wiley interdisciplinary reviews. RNA. Volume 2, number 1, 2011 Jan-Feb, pp. 1–21, doi : 10.1002 / wrna.57 . PMID 21318072 . PMC 3035417 (free full text).
  2. ↑ M. Girardot, J. Cavaillé, R. Feil: Small regulatory RNAs controlled by genomic imprinting and their contribution to human disease. In: Epigenetics: official journal of the DNA Methylation Society. Volume 7, Number 12, December 2012, pp. 1341-1348, doi : 10.4161 / epi.22884 . PMID 23154539 . PMC 3528689 (free full text).
  3. ↑ MS Scott, M. Ono: From snoRNA to miRNA: Dual function regulatory non-coding RNAs. In: Biochemistry. Volume 93, Number 11, November 2011, pp. 1987-1992, doi : 10.1016 / j.biochi.2011.05.026 . PMID 21664409 . PMC 3476530 (free full text).
  4. ↑ C. Verheggen, E. Bertrand: CRM1 plays a nuclear role in transporting snoRNPs to nucleoli in higher eukaryotes. In: Nucleus (Austin, Tex.). Volume 3, Number 2, March 2012, pp. 132-137, doi : 10.4161 / nucl . 19266 . PMID 22555597 . PMC 3383567 (free full text).