Group II intron activities. (A) The splicing reaction. Splicing is intrinsically RNA-catalyzed and occurs for naked RNA in vitro; however, under physiological conditions, the IEP is required as well. The IEP binds to the RNA structure to enable it to adopt its catalytic conformation and accomplish splicing. In the first transesterification step of splicing, the 2′OH of the branch site adenosine initiates nucleophilic attack on the 5′ splice junction, yielding cleaved 5′ exon and a lariat-3′ exon intermediate. In the second transesterification, the 3′ OH of the 5′ exon attacks the 3′ splice site to form ligated exons and intron lariat. The IEP remains tightly bound to the lariat to form a mobility-competent RNP particle. (B) The mobility reaction, known as target-primed reverse transcription (TPRT). The RNP product of splicing recognizes the DNA target site and reverse splices into the top strand. The En domain cleaves the bottom strand and the free 3′ OH is the primer for reverse-transcription. Host repair activities, which vary across organisms, complete the process. IEP, intron-encoded protein.