Structural Biochemistry/Nucleic Acid/RNA/Other RNA

1. Small nuclear RNA (snRNA)

Small nuclear RNAs (snRNA) are the small RNA molecules that are found in the nucleus of eukaryotic cells. They are usually 300 nucleotides or smaller and the nucleus contains more than just snRNA. The function of snRNA was discovered before the ribozyme enzyme by a few years. They are transcribed by RNA polymerase II or RNA polymerase III. They are important because they help in the process of pre-mRNA splicing and processing, which is the removal of the introns from hnRNA, and involved in the maintenance of the telomeres, or the ends of chromosomes. 5 snRNAs makes up the spliceosomes which are responsible for removing the introns from nuclear pre-mRNA eukaryotes. The spliceosome interacts with the ends of an RNA intron. It cuts at specific points to release the intron, then immediately joins the two exons that were adjacent to the intron. They are also responsible for mediating catalysis and aligning splice sites. Thomas Cech and Sydney Altman discovered that RNA molecules can serve as catalysts and changed the views of molecular evolution. snRNA are always found with specific proteins which make up the complexes called small nuclear ribonucleoproteins (snRNP), or snurps. The secondary structures are highly conserved in organisms ranging from yeast and human beings. Large groups of snRNA are called small nucleaolar RNA’s (snoRNA’s). snoRNA’s are responsible for cleaving eukaryotic long preRNA. They are important in RNA biogenesis and guide chemical modifications or ribosomal RNA (rRNA) and other RNA genes (tRNA and snRNA). Many snoRNA’s are created by processed introns. The host gene for the snoRNA is a ribosomal protein or translation factor. [1] [2]

2. Small Nucleolar RNA molecule (snoRNA)

snoRNAs, or Small Nucleolar RNA, modifies ribosomal RNAs (rRNAs) by mediating the cleavage of long pre-rRNA strands into its functional subunits (18S, 5.8S and 28S molecules). snoRNAs can also add the finishing modifications to rRNA subunits. [3]

3. Micro RNA (miRNA)

Micro RNA (miRNA) is a gene regulatory small RNA that is typically 21-23 nucleotides long. It is similar to small interfering RNA (siRNA)in that they bind to complementary mRNA molecules and inhibit their translation, however unlike siRNA which is a double strand RNA, miRNA is a single stranded RNA and it is only partially complementary to mRNA molecules. This class of RNA is non-coding.

Micro RNA has a great range of functions. It is used in cellular growth, development and insulin secretions among other things.

However it has been found that too much miRNA has been found to implicate diseases, such as Fraglie X Mental Retardation, as well as some forms of cancer.

4. Small interfering RNA (siRNA)

siRNAs, are known by several different names: small interfering RNA, short interfering RNA, and silencing RNA, were discovered by David Baulcombe’s research group in Norwich, England. They are roughly 20-25 nucleotides in length and are double stranded RNA molecules with overhanging 2 nucleotides on the 3'ends. They are largely responsible for the process of RNA interference (RNAi) pathways, which interferes with the expression of a gene. Other RNAi pathways such as antiviral mechanisms and shaping the chromatic structure of a genome are also mediated by siRNA. The discovery of siRNA’s ability to be synthetically produced, allowed for the induction of RNAi in mammalian cells. This then allows for research in drug development of the biomedical field such as treatment for the cure of Human Immunodeficiency virus (HIV). However using RNAi through the use of siRNA in living animals is difficult, because siRNA responds differently to different types of cells and the effectiveness varies from very well to poor. It is not yet understood why the effectiveness of siRNA in living animals varies so vastly. Artificial siRNA can be made synthetically by a phage enzyme which is called a dicer. It is the dicer enzyme that causes destruction of the double stranded RNA (dsRNA). By transfecting artificial siRNAs, specific transcripts are used to probe gene function. Although this is a useful tool, the high cost of production makes it nearly impossible for most laboratories and researchers to be able to use this method of probing gene functions by transfecting artificial siRNAs. Chemical synthesis, invitro transcription, or RNase 3-dicer digestion of long dsRNA’s (double stranded RNA’, in vivo from plasmids PCR cassettes, or viral vectors CMV or polymerase III transcription unit. SiRNA’s are used for loss of function studies. SiRNA’s are very sequence specific.

5. RNA Interference (RNAi)

RNAi was discovered Craig Mello and Andrew Fire in the 1990s. The experimented by antisense RNA experiments. It is the process in which double stranded RNA triggers the degradation of homologous mRNA.

Process

RNA interference occurs when a double strand of RNA is broken down by an enzyme called Dicer. Dicer chops the double stranded RNA into short sequences 20-25 base pairs long. These base pairs then complex with the RISC enzyme and a homologous strand of RNA, which is then catalytically cleaved by RISC.

Uses

It is used to degrade mRNA in cells as a defense mechanism against Viral DNA that may have infected the cell and to shut down the effects of specific genes post transcription without having to regulate actual gene expression in the cell's DNA. This can be also used as a gene silencing technique. siRNA is put into a cell by transfection reagents. These reagents increase amount of RNA and DNA that can be absorbed by cultured cells. RNAi is used in the biomedical field to silencing disease causing genes. The RNAi can either be injected into specific cells or using modified viruses to transfect the cells. One common use of RNAi is in the birth control pill which stops sperm from fertilizing the eggs by splicing the gene that encodes protein to allow the sperm to bind to the egg. RNAi is also being used to knock out genes in salamanders in an attempt to discover which genes are responsible for their regenerative capabilities in an attempt to cure diseases previously thought to be incurable, such as Huntington's, Parkinsons, and Alzheimers by attempting to trigger the regeneration of the neurons whose death are responsible for such a disorder.

6. Interference RNA (iRNA)

Interference RNA or iRNA is used for gene regulation. It is an antisense RNA (complementary to other RNA, mostly mRNA). It is important for gene regulation and it is being researched currently for collective anti-cancer properties. It has ties to siRNA as siRNA is involved in the RNA interference pathway. RNA interference (RNAi) is a phenomenon of gene silencing at the mRNA level offering a quick and easy way to determine the function of a gene both in vivo and in vitro. [4]

7. RNA is a component of telomerase

Telomerase is a ribonucleoprotein (a ribonucleic acid-protein complex). It is an enzyme that maintains the telomeres (ends) of chromosomes during DNA replication. It has been found to be useful in the therapeutical, pharmaceutical, and diagnostic reagents.

8. Non-coding RNA (ncRNA)

Non-coding RNA is basically any RNA molecule that is not translated into a protein. Non-coding RNA can be found in many different forms of RNA, such as: ribosomal RNA (rRNA), transfer RNA (tRNA), and small RNAs [microRNA and small interfering RNA (siRNA)]. Non-coding RNA can be small or it can be very large. The small non-coding RNA molecules is also known as sRNA, whereas the large or long non-coding RNA is also known as lncRNA. The non-coding RNA molecule that was transcribed from DNA is often referred to as an RNA gene.

It is significant to note that there exists a growing interest in small, barely detectable non-coding RNA molecules because some of them have been found to play an important role in the regulation of gene expression. These small RNA molecules are known as RNA genes. In the early 1990s, American geneticist Victor Ambros and his colleagues first identified these molecules in the species of worm Caenorhabditis elegans. They were found to be responsible for turning off gene expression during worm development. This novel function was later discovered in other species as well. A decade later, another American geneticist Stephen R. Holbrook of Lawrence Berkeley National Laboratory in California discovered several other potential RNA genes previously undetected via a complex computer program called RNAGENiE. Currently, much research is being conducted over these tiny non-coding RNA molecules. In recent years, biotech and pharmaceutical companies have been looking into the potential of RNA genes as drug targets due to recent interest in RNA genes produced during bacterial infections and their pathogenic effects through the regulation of gene expression of host DNA.

9. Antisense RNA

Antisense RNA is an RNA strand that is complementary to the messenger RNA (mRNA) strand that transcribes within the cell. The antisense RNA is a single stranded RNA molecule. The antisense strand is brought into a cell in order to inhibit the translation of the mRNA. It does this through base pairing to the complementary mRNA strand, which obstructs the ability of the mRNA to translate.

Antisense RNA has been previously thought to be useful as a therapeutic technique for disease therapy, however over the past few years only one drug has been synthesized through the use of antisense RNA. It has been found that antisense RNA failed to have an effective design for disease therapy.

10. tmRNA

In an RNA, an RNAse can take off the 3’ end of an mRNA so that the mRNA has no stop codon for the ribosome sense and stop translation. Once the entire strand of mRNA is translated, this leads to the ribosome being stuck on the mRNA, with a peptidyl-tRNA in the P site of the ribosome. To fix this there is the tmRNA, which removes ribosomes that are stuck on an mRNA. This tmRNA has characteristics of both a tRNA and a mRNA.

In E. Coli, the tmRNA present is SsrA. The structure of this SsrA is arranged so that at one end there is an alanine attached with a tag sequence, and the SsrA is folded to look like a tRNA. The SsrA will enter the A site of the stuck ribosome and the Alanine on the SsrA will form a peptide bond with the polypeptide that is stuck on the ribosome. The tag sequence on the tmRNA is then translated like a mRNA and added to the amino acids on the stuck polypeptide. The string of about 12 added amino acids are called a proteolysis tag. At the end of the tmRNA, a stop codon will signal the ribosome to stop translation and detach itself as well as the SsrA-tagged peptide. SspB, a helper protein, can then recognize the proteolysis tag on the polypeptide chain and bring it to the protease, ClpXp, to be destroyed.

11. Catalytic RNA

Catalytic RNA carry out enzymatic reactions. Catalytic RNAs are usually found near proteins where the catalytic activity is found in the RNA portion, rather than protein.

References

http://science.jrank.org/pages/5892/RNA-Function.html