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 we ll understood at present. The influence of ribosomal RNA conformation on splicing of rDNA introns in vivo was investigated using a heterolog ous system in which the Tetrahymena group I intron is inserted into th e homologous position of the Escherichia coli 23S rRNA. Mutations that block splicing in E. coli result in accumulation of unspliced 23S rRN A that is assembled into 50S complexes, but not 70S ribosomes. The dat a indicate that accommodation of the intron structure on the surface o f the 50S subunit inhibits interactions with the small ribosomal subun it. Spliced intron RNA also remains noncovalently bound to 50S subunit s on sucrose gradients. This interaction appears to be mediated by bas e 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 pi helix. Association of the intron with 50S su bunits correlates with slow cell growth. The results suggest that grou p I introns have the potential to inhibit protein synthesis in prokary otes by direct interactions with ribosomes.