REVERSE SPLICING OF THE TETRAHYMENA IVS - EVIDENCE FOR MULTIPLE REACTION SITES IN THE 23S RIBOSOMAL-RNA

Group I introns in rRNA genes are clustered in highly conserved region s 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 int ervening sequence, IVS) into targe subunit rRNA via reverse splicing w as investigated using E. coli 23S rRNA as a model substrate. The resul ts show that sequences homologous to the splice junction in Tetrahymen a are the preferred site of integration, but that many other sequences in the 23S rRNA provide secondary targets. Like the original splice j unction, many new reaction sites are in regions of stable secondary st ructure. Reaction at the natural splice junction is observed in 50S su bunits 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.