title: Group I introns as mobile genetic elements: facts and mechanistic speculations--a review.
abstract: Group I introns form a structural and functional group of introns with widespread but irregular distribution among very diverse organisms and genetic systems. Evidence is now accumulating that several group I introns are mobile genetic elements with properties similar to those originally described for the omega system of Saccharomyces cerevisiae: mobile group I introns encode sequence-specific double-strand (ds) endoDNases, which recognize and cleave intronless genes to insert a copy of the intron by a ds-break repair mechanism. This mechanism results in: the efficient propagation of group I introns into their cognate sites; their maintenance at the site against spontaneous loss; and, perhaps, their transposition to different sites. The spontaneous loss of group I introns occurs with low frequency by an RNA-mediated mechanism. This mechanism eliminates introns defective for mobility and/or for RNA splicing. Mechanisms of intron acquisition and intron loss must create an equilibrium, which explains the irregular distribution of group I introns in various genetic systems. Furthermore, the observed distribution also predicts that horizontal transfer of intron sequences must occur between unrelated species, using vectors yet to be discovered.
authors.affiliation: Unité de Génétique Moléculaire des Levures, Institut Pasteur, Paris, France.
authors.forename: B
authors.initials: B
authors.lastname: Dujon
mesh_terms: D001483:Base Sequence; D002696:Chlamydomonas / Q000235:genetics; D004251:DNA Transposable Elements; D004706:Endodeoxyribonucleases / Q000235:genetics / Q000378:metabolism; D005656:Fungal Proteins / Q000235:genetics; D007438:Introns; D009690:Nucleic Acid Conformation; D009713:Nucleotidyltransferases / Q000235:genetics; D010804:Physarum / Q000235:genetics; D012326:RNA Splicing; D012331:RNA, Fungal / Q000235:genetics; D012367:RNA, Viral / Q000235:genetics; D012441:Saccharomyces cerevisiae / Q000235:genetics; D013604:T-Phages / Q000235:genetics; D019895:Transposases
chemical_list: D004251:DNA Transposable Elements; D005656:Fungal Proteins; D012331:RNA, Fungal; D012367:RNA, Viral; D009713:Nucleotidyltransferases; D019895:Transposases; D004706:Endodeoxyribonucleases