Introduction

           Group II introns fold into a conserved secondary structure consisting of six domains arranged around a central wheel. Domain 1 is the largest domain by far, while domain 5 is most conserved in sequence and is considered the active site of the ribozyme. 

            For introns that encode ORFs, the ORF is located in the loop of domain 4 and codes for a reverse transcriptase (RT) or related protein. The ORF contains the seven domains common to all RTs, and also domain X and the En domain. Domain X is probably analogous to the thumb domain of other polymerases, and is associated with the maturase (splicing) activity of the protein. The D domain is a DNA binding region, and the En domain has a nuclease activity utilized in the mobility reaction.