An exon is the portion of a gene that codes for amino acids. In the cells of plants and animals, most gene sequences are broken up by one or more DNA sequences called introns. The parts of the gene sequence that are expressed in the protein are called exons, because they are expressed, while the parts of the gene sequence that are not expressed in the protein are called introns, because they come in between--or interfere with--the exons. Exons are that part of the RNA that code for proteins. And really, the important parts of that RNA are the exons.
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Of all spliceosome complexes, the only one lacking a resolved structure is the first to assemble on the pre-mRNA, known as the E complex. It remains unclear how the spliceosome initially defines recognizes and assembles across introns or exons, and how canonical splicing is favoured over a non-canonical reaction known as back-splicing, which generates exonic circular RNAs circRNAs.
Li et al. In yeast, which typically contain short introns and long exons, intron definition seems to dominate. By contrast, in vertebrates, where introns are longer and exons are shorter, exon definition is thought to prevail. Thus, secondary structures may help to bring together essential intronic elements and facilitate spliceosome assembly. If splicing of DYN2 is governed solely by intron definition, retention of the intron in which a mutation resides would be expected, with minimal effect on the other intron; however, if splicing is governed by exon definition, each of the mutations would lead to the retention of both introns.
The observed composition of splicing intermediates produced from the different IEI mutants suggested that both intron definition and exon definition occur in vivo and contribute to correct splicing of DYN2 and showed, for the first time, that exon definition occurs in yeast. Shortening exons of circRNA-producing IEI constructs abolished circRNA formation from the constructs, supporting the preference for back-splicing across long or multiple exons and the occurrence of exon definition.
In summary, in yeast and likely in all eukaryotes the same E complexes are able to define both introns and exons, without the need for additional components or structural rearrangements.
Furthermore, exon definition can cause back-splicing across long exons, suggesting that circRNAs are natural by-products of spliceosome-mediated splicing in all eukaryotes. Li, X. A unified mechanism for intron and exon definition and back-splicing. Shi, Y. Mechanistic insights into precursor messenger RNA splicing by the spliceosome. Cell Biol. Download references. Correspondence to Eytan Zlotorynski. Reprints and Permissions. Zlotorynski, E. Intron definition, exon definition and back-splicing revisited.
Nat Rev Mol Cell Biol 20, Download citation. Published : 23 September Issue Date : November International Journal of Molecular Sciences Advanced search. Skip to main content. Register your interest.
Subjects RNA splicing. Download PDF. References Original article Li, X. You can also search for this author in PubMed Google Scholar. Rights and permissions Reprints and Permissions. About this article. Cite this article Zlotorynski, E. Nature Reviews Molecular Cell Biology menu. Nature Research menu. Search Article search Search. Close banner Close. Enter your email address. Sign up. Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing.
Intron definition, exon definition and back-splicing revisited
Calreticulin proteins CRTs are important components of tick saliva, which is involved in the blood meal success, pathogen transmission and host allergic responses. The characterization of the genes encoding for salivary proteins, such as CRTs, is pivotal to understand the mechanisms of tick-host interaction during blood meal and to develop tick control strategies based on their inhibition. In hard ticks, crt genes were shown to have only one intron with conserved position among species. In this study we investigated the exon-intron structure and variation of the crt gene in Rhipicephalus spp.
A unified mechanism for intron and exon definition and back-splicing
By definition, exons and introns are sequences in a protein-coding gene region of a double-stranded DNA molecule dsDNA that are ex pressed as proteins, or int ervening sequences not so expressed. The exons and introns are typically shown as the single-stranded sequences of the Sense Strand of the dsDNA , written 5'-3' , left to right. The RNA sequences equivalent to the DNA exons and introns are sometimes themselves referred to as " exons " and " introns ," however this is technically incorrect and also confuses their functional role in transcription and translation with exons and introns as gene sequences in DNA. The RNA sequences equivalent to to DNA exons and introns can be referred to as " exon transcripts " and " intron transcripts ," or " equivalents ," respectively. Processing of the hnRNA to mRNA involves excision ' splicing out ' of the intron transcripts and ligation of the remaining exons.
Sigma receptor is a protein that interacts with a variety of psychotomimetic drugs including cocaine and amphetamines and is believed to play an important role in the cellular functions of various tissues associated with the endocrine, immune, and nervous systems. Here we report on the structure and organization of the human gene coding for this receptor. The gene is approximately 7 kbp long and contains four exons, interrupted by three introns. Exon 3 is the shortest 93 bp , and exon 4 is the longest 1, bp. Among the introns, intron 3 is the longest approximately 1, bp.