SnoRNAs – tricky little suckers

SnoRNAs  (small nucleolar RNAs) are a class of small RNAs found in eukaryotes (like us) and archaea (those strange bacteria-like greeblies that live in extreme environments). They are about 100-200 nucleotides in length, come in a couple of flavours depending on their secondary structure and small motifs and can do a variety of things. In fact there are some snoRNAs that we just don’t know what they do. In general however, they modify or guide the modification other RNAs especially rRNAs, snRNAs and tRNAs. The modifications include pseudouridylation (try saying that after a few beers) and 2’-O- methylation.  Basically the chemical structure of the nucleotide is altered slightly giving that nucleotide different chemical function. Nucleotide modification can give increased thermal stability to an RNA structure which is important since RNAs can be greatly affected by temperature. We still don’t know a lot about how these modifications and how they aid the cell, only that they do.

One group of snoRNAs are the C/D box snoRNAs, named after C and D motifs contained in the RNA. These snoRNAs bind to 4 proteins then bind to the target RNA to guide 2’-O-methyationAnother group of snoRNAs have H/ACA motifs and function to direct the conversion of uridines (U’s – the RNA equivalent to T in DNA) to pseudouridine. Just to make life more fun there are some snoRNAs that have both C/D and H/ACA motifs and function in both types of modification.

It is those snoRNA that do not have a target modification site that are gaining interest with researchers. Some C/D box snoRNAs are involved in the process of imprinting where only one copy of the gene from each parent is turned on. The copy to be turned off is targeted by the snoRNA, and promoter regions that turn the gene on are methylated so they don’t work.  And it appears that these snoRNAs can get lonely.  A whole cluster of snoRNAs containing 29 copies SNORD116 (HBII-85) and 48 copies of SNORD115 (HBII-52) has been implicated in Prader-Willi syndrome (a genetic disorder).

Not all snoRNAs guide modification but direct cleavage instead, and it is likely that as we investigate further we will find yet again that RNA-protein complexes do more than we think.  We will likely find that there are even more RNA of all sizes (short, medium and long) connecting together in the tangled web of RNA networks. We can be comforted that these RNAs know what they are doing, even if as yet we don’t know all of what they do.   

They are truly multifunctional molecules!


For some general information:   http://en.wikipedia.org/wiki/Small_nucleolar_RNA

SnoPatrol: how many snoRNA genes are there? Paul P Gardner, Alex Bateman and Anthony M Poole  http://www.biomedcentral.com/content/pdf/jbiol211.pdf


 http://biochem.ncsu.edu/faculty/maxwell/Research.htm