Reverse splicing of the Tetrahymena IVS: evidence for multiple reaction sites in the 23S rRNA.

Group I introns in rRNA genes are clustered in highly conserved regions that include tRNA and mRNA binding sites. This pattern is consistent with insertion of group I introns by direct interaction with exposed regions of rRNA. Integration of the Tetrahymena group I intron (or intervening sequence, IVS) into large subunit rRNA via reverse splicing was investigated using E. coli 23S rRNA as a model substrate. The results show that sequences homologous to the splice junction in Tetrahymena are the preferred site of integration, but that many other sequences in the 23S rRNA provide secondary targets. Like the original splice junction, many new reaction sites are in regions of stable secondary structure. Reaction at the natural splice junction is observed in 50S subunits and to a lesser extent in 70S ribosomes. These results support the feasibility of intron transposition to new sites in rRNA genes via reverse splicing.