Catalytic RNA has a guanosine-bin ding site 23.4 Ribozymes have sub strate-b inding site various catalytic Guanosine bind in CUCUC Sub strate bind activities ACC First transfer G-OH occupies G-binding site; 5 e xon occupies substrate-binding site Figure 23. 6 Excision of the group i intron in Tetrahymena IRNA occurs by successive Second transfer is in G-binding site: 5 reactions between the occupants exon is in substrate-bind ing site 5-CUCUC of the guanosine-binding site and substrate-binding site. The left ACe GAC exon is red, and the right exon is Third transfer G is in G-binding site: 5'end of intron is in sub strate-binding site purple GGGACO GGGAGG 消当 UUUACCUFigure 23.6 Excision of the group I intron in Tetrahymena rRNA occurs by successive reactions between the occupants of the guanosine-binding site and substrate-binding site. The left exon is red, and the right exon is purple. 23.4 Ribozymes have various catalytic activities