G-0H3 ent exon 3 Left exon Right exon 5 CUCUCU PA 5′圆 CUCUCUpU3 GGGAGG Spliced exons pUT tron GGGAGG G 5 CUCUCU-OH ApUUU GGGAGG The sequence of reactions in th elf-splicing of Tetrahymena pre-rRNA.(1)The 3-OH group of a guanine nucleotide attacks the GGGAGG intron's5'-terminal phosphate to form a phosphodiester bond, displacing the 5exon (2) The newly generated 3-oH group of the 5 exon attacks the 5'-terminal phosphate of the 3 Cyclized intron exon, thereby splicing the two exons and displac ing the intron. (3)The 3-OH group of the intron attacks the phosphate of the nucleotide that is 15 residues from the 5 end so as to cyclize the in- UUU-OH tron and displace its 5'-terminal fragment Throughout this process, the RNA maintains a folded, internally hydrogen-bonded conformation that permits the precise excision of the intron