Small Nuclear RNA - Classes of SnRNA

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.