Schematic representation of the two processing pathways in nuclear group I introns. The splicing pathway (left) is initiated by binding of exogenous guanosine (exoG) to the intron internal G binding site followed by nucleophilic attack at the 5′ SS presented in the P1 context. Two transesterification reactions referred to as the first and second steps of splicing result in a spliced out intron with the exoG attached at the 5′ end and spliced exons. The spliced out intron can undergo one of two different sets of reactions. In twin-ribozyme introns (e.g., Dir.S956-1), a set of reactions leads to the formation of an mRNA encoding a homing endonuclease. In most nuclear group I introns (e.g., Tth.L1925), the ωG attacks an internal phosphodiester bond close to the 5′ end presented in a way analogous to the P1 presentation of the 5′ SS. As a result, shortened circles are formed (IGS circles). The circularization pathway (right) is initiated by hydrolysis at the 3′ SS followed by nucleophilic attack of ωG at the 5′ SS presented in the P1 context. This pathway involves one site-specific hydrolysis reaction and one transesterification and results in the formation of full-length intron circles, leaving the exons unligated.
