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I-CreI
I-''Cre''I is a homing endonuclease whose gene was first discovered in the chloroplast genome of ''Chlamydomonas reinhardtii'', a species of unicellular green algae. It is named for the facts that: it resides in an Intron; it was isolated from ''Clamydomonas reinhardtii''; it was the first (I) such gene isolated from ''C. reinhardtii''. Its gene resides in a group I intron in the 23S ribosomal RNA gene of the ''C. reinhardtii'' chloroplast, and I-''Cre''I is only expressed when its mRNA is spliced from the primary transcript of the 23S gene. I-''Cre''I enzyme, which functions as a homodimer, recognizes a 22-nucleotide sequence of duplex DNA and cleaves one phosphodiester bond on each strand at specific positions. I-''Cre''I is a member of the LAGLIDADG family of homing endonucleases, all of which have a conserved LAGLIDADG amino acid motif that contributes to their associative domains and active sites. When the I-''Cre''I-containing intron encounters a 23S gene lacking the intron, I-''Cre''I enzyme "homes" in on the "intron-minus" allele of 23S and effects its parent intron's insertion into the intron-minus allele. Introns with this behavior are called mobile introns. Because I-''Cre''I provides for its own propagation while conferring no benefit on its host, it is an example of selfish DNA.
==Discovery==
I-''Cre''I was first observed as an intervening sequence in the 23S rRNA gene of the ''C. reinhardtii'' chloroplast genome.〔 The 23S gene is an RNA gene, meaning that its transcript is not translated into protein. As RNA, it forms part of the large subunit of the ribosome. An open reading frame coding for a 163-amino acid protein was found in this 23S intron, suggesting that a protein might facilitate the homing behavior of the mobile intron. Furthermore, the predicted protein had a LAGLIDADG motif, a conserved amino acid sequence that is present in other proteins coded for in group I mobile introns. A 1991 study established that the ORF codes for a DNA endonuclease, I-''Cre''I, which selectively cuts a site corresponding to where the intron is spliced out of the 23S primary transcript. The study also showed that the intron was able to invade 23S alleles that did not already have it.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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