The conserved U.G pair in the 5' splice site duplex of a group I intron is required in the first but not the second step of self-splicing.
Barfod ET, Cech TR. The conserved U.G pair in the 5' splice site duplex of a group I intron is required in the first but not the second step of self-splicing. Mol Cell Biol. 1989 Sep; 9(9):3657-66.