Efficient protein-facilitated splicing of the yeast mitochondrial bI5 intron
Abstract
The splicing factor CBP2 is required to excise the yeast mitochondrial group I intron bI5 in vivo and at low magnesium ion concentrations in vitro. CBP2 binding is strengthened 20-fold by increasing Mg2+ concentrations from 5 to 40 mM, implying the protein binds, in part, to the same structure as that stabilized by the cation. The same transition is also observed as a cooperative increase in the rate of self-processing between 5 and 40 mM Mg2+, providing strong evidence for an RNA folding transition promoted by either Mg2+ or CBP2. The first step of splicing, guanosine addition at the 5' splice site, is rate limiting for exon ligation. At low (5 mM) magnesium ion, reaction (measured as kcat/Km or kcat) is accelerated 3 orders of magnitude by saturating CBP2. At near-saturating Mg2+ (40 mM), acceleration is 8- and 30-fold, for kcat and kcat/Km, respectively, so high magnesium ion concentrations fail to compensate completely for protein facilitation. Thus, self-splicing proceeds via two additional transitions as compared with reaction of the bI5-CBP2 complex, only the first of which is efficiently promoted by the cation. Guanosine 5'-monophosphate binds (Kd approximately 0.3 mM) with the same affinity to bI5 and the bI5-protein c...Continue Reading
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