Where are group II introns found?
Mitochondria and chloroplasts
Group II introns are relatively abundant in the organellar genomes of plants, fungi, algae and protists, but remain notably absent from nuclear genomes. In organelles, group II introns are frequently found in highly conserved genes, such as cytochrome oxidase or rubisco subunits. Often there are many introns per gene. Because many organellar introns are ORF-less, they must rely on additional host-encoded factors for their splicing reactions.
Approximately one quarter of sequenced bacterial genomes contain group II introns. The bacterial introns differ from their organellar counterparts in a variety of ways. A large proportion are found intergenically, or inserted within mobile DNAs such as plasmids, IS elements or pathogenicity islands. Only a small proportion is located within conserved housekeeping genes. In addition, ORF-less group II introns are rare and relatively few examples have been identified in bacteria (Simon et al. RNA, 2008). For these and additional reasons, (Dai & Zimmerly, 2002) it appears that group II introns in bacteria behave mainly as retroelements, which is a significant difference from their organellar intron counterparts.
Group II introns have been identified in a few archaeabacterial genomes. The archaeal introns do not form a distinct class of group II introns, but are closely related to introns in eubacteria. Some archaeal introns are inserted within other introns, forming "twintrons" with up to four sets of nested introns. In addition, a number of the archaeal introns do not encode RT ORFs but seem to be mobile nevertheless. (Dai & Zimmerly, 2003)