Small nucleolar RNA
SnoRNA ( small nucleolar ribonucleic acid english, small nucleolar ribonucleic acid ') are RNA in eukaryotes and some archaea involved in the processing and modification of other ribonucleic acids - are involved and play a role in imprinting - in particular ribosomal RNA ( rRNA).
Properties
SnoRNA do not encode proteins but function as guide RNA by bringing the enzymes to the correct locations of the RNA. In the cell are snoRNAs - as well as almost all other RNAs - not naked, but associated with proteins as ribonucleoprotein before, we speak therefore of snoRNPs ( small nucleolar ribonucleoprotein particle english ).
The modifications are introduced by this snoRNPs in the ribosomal RNA are essential for the function of the ribosome in the course of the translation, and which is essential in the snRNAs for splicing. The snoRNPs are - as the name implies - usually found in the nucleoli, where they modify the rRNAs, with the exception of the related scaRNAs ( see below). SnoRNA be funneled into eukaryotes by Crm1 in the nucleus.
In addition to the guide of the RNAs support modifications of nucleic acids there is also a direct modification without the help of this small RNA ( eg, DNA methylation by DNA methyltransferases, methylation of the cap or RNA editing by ADAR enzymes eg ).
SnoRNA families
Generally, one can distinguish two types of snoRNAs, the box C / D and box H / ACA type, each responsible for a different modification of the RNA. The name comes from different sequence or folding motifs in the respective RNAs.
In addition, both types of RNAs associated with different proteins that are involved in the proper recognition of the target and perform 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 methylation of 2'O riboses. They recognize the target RNA due to complementary sequences and bind it to the right place. Then, the catalytic subunit of the protein fibrillarin, active and transfers the methyl group of S -adenosylmethionine to the ribose.
Box H / ACA snoRNPs work basically the same, even here the snoRNA recognizes the target RNA, but the subsequent reaction is no methylation but a conversion of uridine to pseudouridine by the protein dyskerin.
ScaRNAs
More recently, another type was discovered at modification guide RNAs, the so-called scaRNAs (small Cajal body RNAs localized ). These RNAs have some similarities with snoRNAs, however, contain both box C / D and H / ACA motifs, which is why they are also referred to as chimeric RNAs. Unlike the snoRNAs scaRNAs (function nucleäre body discovered about 100 years ago by Ramon y Cajal, probably in recycling and biogenesis spliceosomaler snRNPs ) are in the Cajal Bodies to find (even coiled bodies, CBs ). They also do not catalyze the modification of ribosomal RNAs but which are of snRNAs and thus essential for splicing.