Dual roles for the Mss116 cofactor during splicing of the ai5γ group II intron |
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Authors: | Nora Zingler Amanda Solem Anna Marie Pyle |
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Institution: | 1.Department of Molecular Biophysics and Biochemistry and 2.Howard Hughes Medical Institute, Yale University, New Haven, CT 06520, USA |
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Abstract: | The autocatalytic group II intron ai5γ from Saccharomyces cerevisiae self-splices under high-salt conditions in vitro, but requires the assistance of the DEAD-box protein Mss116 in vivo and under near-physiological conditions in vitro. Here, we show that Mss116 influences the folding mechanism in several ways. By comparing intron precursor RNAs with long (∼300 nt) and short (∼20 nt) exons, we observe that long exon sequences are a major obstacle for self-splicing in vitro. Kinetic analysis indicates that Mss116 not only mitigates the inhibitory effects of long exons, but also assists folding of the intron core. Moreover, a mutation in conserved Motif III that impairs unwinding activity (SAT → AAA) only affects the construct with long exons, suggesting helicase unwinding during exon unfolding, but not in intron folding. Strong parallels between Mss116 and the related protein Cyt-19 from Neurospora crassa suggest that these proteins form a subclass of DEAD-box proteins that possess a versatile repertoire of diverse activities for resolving the folding problems of large RNAs. |
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