Introduction

            Group II introns are relatively abundant in organellar genomes of plants and lower eukaryotes, but have not yet been found in higher eukaryotes or in nuclear genomes. In organelles, group II introns are typically found in highly conserved genes such as cytochrome oxidase or rubisco subunits, and there are often many introns in a gene. 

Bacteria

             In bacteria, about one quarter of genomes contain group II introns. Bacterial introns differ in several respects from the organellar group II introns. The bacterial introns are not located in conserved genes, but instead are located in mobile DNAs such as plasmids, IS elements or pathogenicity islands. The introns are often inserted outside of ORFs, and almost all introns identified so far encode reverse transcriptase ORFs and are either active retroelements or derivatives of retroelements. For these and additional reasons (see Dai and Zimmerly, 2002) we have proposed that group II introns in bacteria behave mainly as retroelements, a significant difference from organellar group II introns. 

Archaebacteria

             Group II introns are now known in archaebacteria, where they are found in several related Methanosarcina species. The archael introns are closely related to group II introns in eubacteria, but there are differences in their insertion patterns. None of the archael introns are located in cellular genes, but are instead inserted into other introns, forming “twintrons” with up to four sets of nested introns. Moreover, a number of the introns do not encode RT ORFs but seem to be mobile nevertheless. (See Dai & Zimmerly 2003 for detailed information)