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An intein is a segment of a protein that is able to excise itself and join the remaining portions (the exteins) with a peptide bond in a process termed protein splicing. Inteins have also been called "protein introns". Intein-mediated protein splicing occurs after the intein-containing mRNA has been translated into a protein. This precursor protein contains three segments — an N-extein followed by the intein followed by a C-extein. After splicing has taken place, the resulting protein contains the N-extein linked to the C-extein; this splicing product is also termed an extein. == History == The first intein was discovered in 1988 through sequence comparison between the ''Neurospora crassa'' and carrot vacuolar ATPase (without intein) and the homologous gene in yeast (with intein) that was first described as a putative calcium ion transporter. In 1990 Hirata ''et al.'' demonstrated that the extra sequence in the yeast gene was transcribed into mRNA and removed itself from the host protein only after translation. Since then, inteins have been found in all three domains of life (eukaryotes, bacteria, and archaea) and in viruses. Most reported inteins also contain an endonuclease domain that plays a role in intein propagation. In fact, many genes have unrelated intein-coding segments inserted at different positions. For these and other reasons, inteins (or more properly, the gene segments coding for inteins) are sometimes called ''selfish genetic elements'', but it may be more accurate to call them parasitic. According to Dawkins' gene centered view of evolution, all genes are selfish, even those that cooperate with other genes to build the organisms that carry the genes into the next generation. Most genes are "selfish" only insofar as to compete with other genes or alleles but usually they fulfill a function for the organisms, whereas "parasitic genetic elements", at least initially, do not make a positive contribution to the fitness of the organism. Within the database of all known inteins ((Inbase )), 113 known inteins are present in eukaryotes with minimum length of 138 amino acids and maximum length of 844 amino acids. The first intein was found encoded within the VMA gene of ''Saccharomyces cerevisiae''. They were later found in fungi (ascomycetes, basidiomycetes, zygomycetes and chytrids) and in diverse proteins as well. A protein distantly related to known inteins containing protein, but closely related to metazoan hedgehog proteins, has been described to have the intein sequence from Glomeromycota. Many of the newly described inteins contain homing endonucleases and some of these are apparently active. The abundance of intein in fungi indicates lateral transfer of intein-containing genes. While in eubacteria and archea, there are 289 and 182 known inteins until this literature review was written. Not surprisingly, most intein in eubacteria and archea are found to be inserted into nucleic acid metabolic protein, like fungi.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「intein」の詳細全文を読む スポンサード リンク
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