An mRNA Is Capped by a 2', 5' Lariat Catalyzed by a Group I-Like Ribozyme

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Science  02 Sep 2005:
Vol. 309, Issue 5740, pp. 1584-1587
DOI: 10.1126/science.1113645

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Twin-ribozyme introns are formed by two ribozymes belonging to the group I family and occur in some ribosomal RNA transcripts. The group I-like ribozyme, GIR1, liberates the 5′ end of a homing endonuclease messenger RNA in the slime mold Didymium iridis. We demonstrate that this cleavage occurs by a transesterification reaction with the joining of the first and the third nucleotide of the messenger by a 2′,5′-phosphodiester linkage. Thus, a group I-like ribozyme catalyzes an RNA branching reaction similar to the first step of splicing in group II introns and spliceosomal introns. The resulting short lariat, by forming a protective 5′ cap, might have been useful in a primitive RNA world.

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