Introduction

           Group II introns are a novel class of RNAs best known for their self-splicing reaction. Under certain in vitro conditions, the introns can excise themselves from precursor mRNAs and ligate together their flanking exons, without the aid of protein. The splicing mechanism is essentially identical to splicing of nuclear pre-mRNA introns, and this similarity has led to the widespread belief that group II introns were the ancestors of spliceosomal introns, which make up 25% of the human genome. 

            Some group II introns have a second remarkable property: they encode reverse transcriptase (RT) ORFs and are active mobile elements. Such mobile group II introns can insert into defined sites at high efficiencies (called retrohoming), or can invade unrelated sites at low frequencies (retrotransposition).

            Group II introns were discovered and first studied in organellar genomes, where they are relatively abundant. However, group II introns are now being found in unexpected numbers in bacterial genomes, a consequence of the genome sequencing projects. Many newly sequenced bacterial group II introns are incorrectly identified or annotated. A major goal of this web site is to be a resource in helping to identify and catalog group II introns in the databases.