17 Group II Introns: Ribozymes That Splice RNA and Invade DNA

Abstract

Group II introns are a distinct class of ribozymes that catalyze their own excision from precursor RNAs (Michel and Ferat 1995; Bonen and Vogel 2001; Lehmann and Schmidt 2003). Once released, the intron RNAs are highly reactive transposable elements that can insert themselves, via reverse splicing, into new genomic locations, where they are converted to DNA by reverse transcription (Belfort et al. 2002; Lambowitz and Zimmerly 2004). Group II intron RNAs generally consist of six domains (DI–DVI), one of which (DIV) often contains an open reading frame (ORF) that encodes a specialized helper protein, called a “maturase” (Fig. 1) (Michel et al. 1989). The intron-encoded protein (IEP) stabilizes the active RNA structure and provides reverse transcriptase (RT) activity, thus assisting in both splicing and intron mobility. However, the catalytic apparatus for forward and reverse splicing resides within the RNA (Lambowitz et al. 1999).

Group II introns are found in eubacteria (Ferat and Michel 1993; Martínez-Abarca and Toro 2000a), archaea (Toro 2003), and the organellar genes of plants, fungi, and yeast (Bonen and Vogel 2001). Different structural subclasses of group II intron RNAs (IIa, IIb, and IIc) have been distinguished on the basis of variations in the conserved structure and are thought to represent distinct lineages of mobile introns (Michel et al. 1989; Toor et al. 2001; Toro 2003). Group II introns provide an important window into basic understanding of RNA catalysis because they promote a variety of chemical reactions, often in collaboration with proteins such as maturases (Lehmann and...