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Science 2 September 2005:
Vol. 309. no. 5740, pp. 1584 - 1587
DOI: 10.1126/science.1113645
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Reports

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

Henrik Nielsen,1,2* Eric Westhof,3 Steinar Johansen2

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.

1 Department of Medical Biochemistry and Genetics, The Panum Institute, University of Copenhagen, DK-2200N Copenhagen, Denmark.
2 Department of Molecular Biotechnology-RNA Research Group, Institute of Medical Biology, University of Tromsø, N-9037 Tromsø, Norway.
3 Institut de Biologie Moléculaire et Cellulaire, CNRS, Université Louis Pasteur, 67084 Strasbourg Cedex, Strasbourg, France.

* To whom correspondence should be addressed. E-mail: hamra{at}imbg.ku.dk

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