Association of a group I intron with its splice junction in 50S ribosomes: implications for intron toxicity.

The effect of genetic context on splicing of group I introns is not well understood at present. The influence of ribosomal RNA conformation on splicing of rDNA introns in vivo was investigated using a heterologous system in which the Tetrahymena group I intron is inserted into the homologous position of the Escherichia coli 23S rRNA. Mutations that block splicing in E. coli result in accumulation of unspliced 23S rRNA that is assembled into 50S complexes, but not 70S ribosomes. The data indicate that accommodation of the intron structure on the surface of the 50S subunit inhibits interactions with the small ribosomal subunit. Spliced intron RNA also remains noncovalently bound to 50S subunits on sucrose gradients. This interaction appears to be mediated by base pairing between the intron guide sequence and the 23S rRNA, because the fraction of bound intron RNA is reduced by point mutations in the IGS or deletion of the P1 helix. Association of the intron with 50S subunits correlates with slow cell growth. The results suggest that group I introns have the potential to inhibit protein synthesis in prokaryotes by direct interactions with ribosomes.