Abstract
The group I-like ribozyme GIR1 is a unique example of a naturally occurring ribozyme with an evolved biological function. GIR1 generates the 5′-end of a nucleolar encoded messenger RNA involved in intron mobility. GIR1 is found as a cis-cleaving ribozyme within two very different rDNA group I introns (twin-ribozyme introns) in distantly related organisms. The Didymium GIR1 (DiGIR1) and Naegleria GIR1 (NaGIR1) share fundamental features in structural organization and reactivity, and display significant differences when compared to the related group I splicing ribozymes. GIR1 lacks the characteristic P1 segment present in all group I splicing ribozymes, it has a novel core organization, and it catalyses two site-specific hydrolytic cleavages rather than splicing. DiGIR1 and NaGIR1 appear to have originated from eubacterial group I introns in order to fulfil a common biological challenge: the expression of a protein encoding gene in a nucleolar context.
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