The presence of a natural rRNA secondary structure element immediately preceding the 5' splice site of the Tetrahymena IVS can Inhibit self-splicing by competing with base pairing between the 5' exon and the guide sequence of the IVS (P1). Formation of this alternative hairpin Is preferred in short precursor RNAs, and results in loss of G-addrtion to the 5' splice site. Pre-rRNAs which contain longer exons of ribosomal sequence, however, splice rapidly. As many as 146 nucleotides of the 5' exon and 86 nucleotides of the 3' exon are required for efficient self-splicing of Tetrahymena precursors. The presence of nucleotides distant from the 5' splice site apparently alters the equilibrium between the alternative hairpins, and promotes formation of active precursors. This effect is dependent on the specific sequences of the ribosomal pre-RNA, since point mutations within this region reduce the rate of splicing as much as 50-fold. This system provides an opportunity to study the way in which long-range interactions can influence splice site selection in a highly structured RNA.
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