3-OH ofG attacks pA or pl OG. UUUpACCUpUUG 5G.. UUUpAC CUpUUG 23.2 Group I OH introns undertake self-splicing by Cyclization GpUUG transesterification Reverse cyclization Figure 23.3 The excised intron can form circles by pUG using either of two internal Lineanzation sites for reaction with the 5 end, and can reopen the L15 RNA L-19 RNA trans reaction circles by reaction with 1416 UG JUpACCUpUUC water or oligonucleotides 消当Figure 23.3 The excised intron can form circles by using either of two internal sites for reaction with the 5 end, and can reopen the circles by reaction with water or oligonucleotides. 23.2 Group I introns undertake self-splicing by transesterification