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Mentions: Group I introns catalyze RNA phosphoester transfer reactions at specific splice sites both in vivo and in vitro. The splice site selection is precise, and relies on base-pairing interactions between the 5′ portion in P1 of the catalytic intron and a pseudo-complementary 3–6-nt region of the 3′ portion of the 5′ exon (1,2). The former has been termed the internal guide sequence (IGS) and the latter can be referred to as the IGS complement. Base-pairing between the IGS and its complement depends on Watson–Crick pairing at most positions; however the 3′ nt of the IGS complement always forms a G•U wobble pair with the IGS to define precisely the 5′ splice site (1–4). For example, the IGS of the group I intron in the tRNAIle transcript from the purple bacterium Azoarcus can be shortened in vitro to 5′-GUG-3′, and this pairs with the complement 5′-CAU-3′ to effect splicing after the terminal U in the complement (5–7) (Figure 1A).Figure 1.
One RNA plays three roles to provide catalytic activity to a group I intron lacking an endogenous internal guide sequence
Bottom Line: However, a single RNA genotype has the potential to adopt two or perhaps more distinct phenotypes as a result of differential folding and/or catalytic activity.Such multifunctionality would be particularly significant if the phenotypes were functionally inter-related in a common biochemical pathway.This property of RNA to be multifunctional in a single reaction pathway bolsters the probability that a system of self-replicating molecules could have existed in an RNA world during the origins of life on the Earth.
Affiliation: Department of Chemistry, Portland State University, PO Box 751, Portland, OR 97207, USA.
Catalytic RNA molecules possess simultaneously a genotype and a phenotype. However, a single RNA genotype has the potential to adopt two or perhaps more distinct phenotypes as a result of differential folding and/or catalytic activity. Such multifunctionality would be particularly significant if the phenotypes were functionally inter-related in a common biochemical pathway. Here, this phenomenon is demonstrated by the ability of the Azoarcus group I ribozyme to function when its canonical internal guide sequence (GUG) has been removed from the 5' end of the molecule, and added back exogenously in trans. The presence of GUG triplets in non-covalent fragments of the ribozyme allow trans-splicing to occur in both a reverse splicing assay and a covalent self-assembly assay in which the internal guide sequence (IGS)-less ribozyme can put itself together from two of its component pieces. Analysis of these reactions indicates that a single RNA fragment can perform up to three distinct roles in a reaction: behaving as a portion of a catalyst, behaving as a substrate, and providing an exogenous IGS. This property of RNA to be multifunctional in a single reaction pathway bolsters the probability that a system of self-replicating molecules could have existed in an RNA world during the origins of life on the Earth.
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