Splicing of group I introns is processed by two sequential transesterification reactions.
The exogenous guanosine or guanosine nucleotide (exoG) first docks onto the active G-binding site located in P7, and its 3'-OH is aligned to attack the phosphodiester bond at the 5' splice site located in P1, resulting in a free 3'-OH group at the upstream exon and the exoG being attached to the 5' end of the intron.
Then the terminal G (omega G) of the intron swaps the exoG and occupies the G-binding site to organize the second ester-transfer reaction: the 3'-OH group of the upstream exon in P1 is aligned to attack the 3' splice site in P10, leading to the ligation of the adjacent upstream and downstream exons and release of the catalytic intron.

Two-metal-ion mechanism seen in protein polymerases and phosphatases was proposed to be used by group I and group II introns to process the phosphoryl transfer reactions, which was unambiguously proven by a high-resolution structure of the Azoarcus group I intron in 2006.
Splicing of group I introns is processed by two sequential transesterification reactions. The exogenous guanosine or guanosine nucleotide (exoG) first docks onto the active G-binding site located in P7, and its 3'-OH is aligned to attack the phosphodiester bond at the 5' splice site located in P1, resulting in a free 3'-OH group at the upstream exon and the exoG being attached to the 5' end of the intron. Then the terminal G (omega G) of the intron swaps the exoG and occupies the G-binding site to organize the second ester-transfer reaction: the 3'-OH group of the upstream exon in P1 is aligned to attack the 3' splice site in P10, leading to the ligation of the adjacent upstream and downstream exons and release of the catalytic intron.
Two-metal-ion mechanism seen in protein polymerases and phosphatases was proposed to be used by group I and group II introns to process the phosphoryl transfer reactions, which was unambiguously proven by a high-resolution structure of the Azoarcus group I intron in 2006.
Group I catalytic intron