snRNA

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The abbreviation snRNA stands for small nuclear ribonucleic acid ("small nuclear ribonucleic acid"). It is about 100 to 300 base pairs of RNA . SnRNA is located in the cell nucleus and is produced by the polymerases RNA polymerase II and III.

snRNP complexes

In the cell nucleus, snRNA molecules associate with several specific proteins to form a complex, the snRNP ( small nuclear ribonucleoprotein particle ; pronounced snurp ). Type 2 introns , which do not aggregate with proteins , are an exception . Because of the high proportion of uracil , the various snRNP species are also called U1-snRNA, U2-snRNA, etc. Five of these snRNPs form the so-called spliceosome . There are up to a million copies of individual snRNPs. However, only four components are essential for the formation of the spliceosome and for the splicing of the pre-mRNA. The spliceosome is made up of five snRNPs (U1, U2, U4, U5 and U6) with five different RNAs. Seven proteins are present in each snRNP. These proteins are called Sm and LSM proteins ( like Sm proteins). They form a ring-shaped structure which surrounds the snRNA at the "Sm site". All other proteins that occur in a snRNP are specific for the respective snRNP type. All U-snRNAs show pronounced secondary structures . While the nucleotide sequence differs greatly in different animal and plant species, the tertiary structure is highly conserved. At the 5 'end of the U1, U2, U4 and U5 snRNA there is an N , N , 7-trimethylguanine cap. A 5'-5'-triphosphate bridge is formed from one to the next nucleotide. These four U-snRNAs are transcribed by RNA polymerase II. The U6 snRNA does not have an N, N, 7-trimethylguanine cap. Instead, a methyl triphosphate can be found at the 5 'end. The U6-snRNA is transcribed by RNA polymerase III. The three snRNP types U1, U2 and U5 consist of one RNA molecule, whereas the U4 and U6 snRNA consist of two RNA molecules that bind to one another via hydrogen bonds.

Functions of the snRNA

As part of the spliceosome, the snRNA is catalytically active. It is responsible for recognizing and splicing the introns of the pre-mRNA contained in the cell nucleus. To this end, SnRNPs bind to conserved sequence sections of the pre-mRNA, which are located at the transition between exon and intron (5 'and 3' splice points). Splicing eliminates the introns that are non-coding in the pre-mRNA. Only the coding exons are retained and are linked to one another to form the mRNA. In addition to splicing, snRNA is involved in many other processes. This includes the regulation of RNA polymerase II and some transcription factors. Furthermore, snRNA is responsible for the integrity of the telomeres of the chromosomes .

The snoRNA

The snoRNA ( small nucleolar ribonucleic acid ) is a large subgroup of the snRNA. snoRNA is involved in the chemical modification of tRNA ( transfer RNA ), rRNA (ribosomal RNA) and snRNA, and is also responsible for RNA production. snoRNA is found in the nucleolus, but also in the cajal bodies of eukaryotes .

literature

  • Rolf Knippers: Molecular Genetics. 9th edition. Thieme publishing house
  • Martin Lützelberger: Pre-mRNA splicing in the fission yeast Schizosaccharomyces pombe: In vivo characterization of the function of the srp2 gene. (Dissertation).

Individual evidence

  1. Yigong Shi: Mechanistic insights into precursor messenger RNA splicing by the spliceosome . In: Nature Reviews Molecular Cell Biology . tape 18 , no. November 11 , 2017, ISSN  1471-0080 , p. 655–670 , doi : 10.1038 / nrm.2017.86 ( nature.com [accessed April 24, 2019]).