Classes of SnRNA
snRNA are often divided into two classes based upon both common sequence features as well as associated protein factors. The first, more widely studied, class is known as Sm-class RNA. Sm-class snRNA are transcribed by RNA polymerase II. The pre-snRNA are transcribed and receive a 5’ monomethylguanosine cap in the nucleus. They are then exported to the cytoplasm for further processing. In the cytoplasm, the snRNA receive 3’ trimming to form a 3’ stem-loop structure, as well as hypermethylation of the 5’ cap through trimethylguanosine. The 3’ stem structure is necessary for recognition by the survival motor neuron (SMN) complex. This complex assembles the snRNA into stable ribonucloproteins (RNPs). The 5’ cap is then required to import the snRNP back into the nucleus. The Sm-class of snRNA includes U1, U2, U4, U4atac, U5, U7, U11, and U12. All of these uridine-rich snRNA with the exception of U7 form the core of the spliceosome. Splicing, or the removal of introns, is a major aspect of post-transcriptional modification, and takes place only in the nucleus of eukaryotes. U7 snRNA has been found to function in histone pre-mRNA processing.
The second class, Lsm-class snRNAs, contain a monomethylphosphate cap and a 3' stem–loop, terminating in a stretch of uridines that form the binding site for a distinct heteroheptameric ring of Lsm proteins. Lsm-class snRNAs are transcribed by RNA polymerase III and never leave the nucleus, in contrast to Sm-class snRNA. There are only two know Lsm-class snRNAs, U6 and U6atac.
Read more about this topic: Small Nuclear RNA
Famous quotes containing the words classes of and/or classes:
“There were three classes of inhabitants who either frequent or inhabit the country which we had now entered: first, the loggers, who, for a part of the year, the winter and spring, are far the most numerous, but in the summer, except for a few explorers for timber, completely desert it; second, the few settlers I have named, the only permanent inhabitants, who live on the verge of it, and help raise supplies for the former; third, the hunters, mostly Indians, who range over it in their season.”
—Henry David Thoreau (1817–1862)
“Of all reformers Mr. Sentiment is the most powerful. It is incredible the number of evil practices he has put down: it is to be feared he will soon lack subjects, and that when he has made the working classes comfortable, and got bitter beer into proper-sized pint bottles, there will be nothing left for him to do.”
—Anthony Trollope (1815–1882)