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Intracellular folding of the Tetrahymena group I intron depends on exon sequence and promoter choice.

The Tetrahymena group I intron splices 20 to 50 times faster in Tetrahymena than in vitro, implying that the intron rapidly adopts its active conformation in the cell. The importance of cotranscriptional folding and the contribution of the rRNA exons to the stability of the active pre-RNA structure were investigated by comparing the activity of minimal pre-RNAs expressed in Escherichia coli. Pre-RNAs containing exons derived from E. coli 23 S rRNA were three to four times more active than the wild-type Tetrahymena pre-RNA. E. coli transcripts of the chimeric E. coli pre-RNA were two to eight times more active than were T7 transcripts. However, the effect of cotranscriptional folding depends on exon sequences. Unexpectedly, the unspliced pre-RNA decays more slowly than predicted from the rate of splicing. This observation is best explained by partitioning of transcripts into active and inactive pools. We propose that the active pool splices within a few seconds, whereas the inactive pool is degraded without appreciable splicing.