A large number of discrete, highly conserved small RNA species are found in eukaryotic cells; some are quite stable. Most of these molecules are complexed with proteins to form ribonucleoproteins and are distributed in the nucleus, the cytoplasm, or both. They range in size from 20 to 1000 nucleotides and are present in 100,000 to 1,000,000 copies per cell, collectively representing less than or equal to 5% of cellular RNA.

Small Nuclear RNAs

 snRNAs, a subset of the small nuclear RNAs (see Table 1), are significantly involved in rRNA and mRNA processing and gene regulation. Of the several snRNAs, U1, U2, U4, U5, and U6 are involved in mRNA splicing, a nuclear process whereby introns are removed from mRNA precursor molecules to gener ate functional, translatable cytoplasmic mRNAs (see Chapter 36). The U7 snRNA is involved in production of the correct 3′ ends of histone mRNA—which lacks a poly(A) tail. 7SK RNA associ ates with several proteins to form a ribonucleoprotein complex, termed P-TEFb, that modulates mRNA gene transcription elon gation by RNA polymerase II.

Table1. Some of the Species of Small-Stable RNAs Found in Mammalian Cells

Large & Small Noncoding Regulatory RNAs: Micro-RNAs (miRNAs), Silencing RNAs (siRNAs), Long Noncoding RNAs (lncRNAs), and Circular RNAs (circRNAs)

One of the most exciting and unanticipated discoveries in the last decade of eukaryotic regulatory biology has been the identification and characterization of regulatory nonprotein coding RNAs (ncRNAs). NcRNAs exist in two general size classes, large (50–1000nt) and small (20–22nt). Regulatory ncRNAs have been described in most eukaryotes.

The small ncRNAs termed miRNAs and siRNAs typically inhibit gene expression at the level of specific protein production by targeting mRNAs through one of several distinct mechanisms. miRNAs are generated by specific nucleolytic processing of the products of distinct genes/transcription units. miRNA precursors, which are 5′-capped and 3′-polyadenylated, usually range in size from about 500 to 1000 nucleotides.

By contrast, siRNAs are generated by the specific nucleolytic processing of large dsRNAs that are either produced from other endogenous RNAs, or dsRNAs introduced into the cell by, for example, RNA viruses. Both siRNAs and miRNAs hybridize via the formation of RNA–RNA hybridization to their targeted mRNAs. To date, hundreds of distinct miRNAs and siRNAs have been described in humans; estimates suggest that there are likely 1000s of human miRNA-encoding genes. Given their exquisite genetic specificity, both miRNAs and siRNAs represent exciting new potential agents for therapeutic drug development. In the laboratory siRNAs are frequently used to decrease or “knock down” specific protein levels (via siRNA homology–directed mRNA degradation), and thus serve as an extremely useful and powerful alternative to gene-knockout technology. Indeed, several siRNA-based therapeutic clinical trials are in progress to test the efficacy of these novel molecules as drugs for treating human disease.

Other exciting recent observations in the RNA realm are the identification and characterization of two classes of larger noncoding RNAs, the circular RNAs (circRNAs) and the long noncoding RNAs, or lncRNAs. Many circRNAs have recently been discovered and characterized. circRNAs appear to be produced by RNA splicing-type reactions from a wide range of pre cursor RNAs, both mRNA precursors and nonprotein coding lncRNA precursors (see following discussion for more infor mation on lncRNAs). Though not an abundant class of RNA molecules in most cells, circRNAs have been detected in all eukaryotes tested, and seem particularly abundant in metazoans. While the functions of circRNAs are still being elucidated they seem to be particularly abundant in cells of the nervous system. Similar to lncRNAs, these molecules likely play important roles in cellular biology by regulating gene expression at multiple levels. LncRNAs, which as their name implies, do not code for protein, and range in size from ~300 to 1000s of nucleotides in length. These RNAs are typically transcribed from the large regions of eukaryotic genomes that do not encode for protein (ie, the mRNA encoding genes). In fact, transcriptome analyses indicate that more than 90% of all eukaryotic genomic DNA may be transcribed at some level. ncRNAs make up a significant portion of this transcription. ncRNAs play many roles ranging from contributing to structural aspects of chromatin to regulation of mRNA gene transcription by RNA polymerase II. Future work will further characterize this important, newly discovered class of RNA molecules.

Interestingly, bacteria also contain small, heterogeneous regulatory RNAs termed sRNAs. Bacterial sRNAs range in size from 50 to 500 nucleotides, and like eukaryotic mi/si/ lncRNAs, control the expression/activity of a large array of distinct genes. sRNAs often repress, but sometimes activate protein synthesis by binding to specific mRNA.

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المزيد

مواضيع ذات صلة

Ribosomal RNA

Messenger RNA

Identification of the src and sis Gene

Cellular Counterparts of Retroviral Oncogenes

Retroviruses and Cancer

Identifying a Nucleotide Change Causing Cancer

Transformation and Oncogenesis by Damaging the Chromosome

Bacterially Induced Tumors in Plants

Phosphorylation and the Rapid Response

Chemotaxis: Methylation and Adaptation

The Energy for Chemotaxis

The Mechanism of Chemotaxis