Distribution and Diversity of Group II Introns

Group Ⅱ introns are found in mitochondria and chloroplasts genomes of plants,fungi,protists and algae,as well as bacterial and archaebacterial genomes.In intracellular organelles,group Ⅱ introns are typically found in highly conserved genes,whereas the bacterial group Ⅱ introns are mainly located in mobile DNAs which are carrying non-conserved genes,except the recent discovery of the presence of group Ⅱ introns in some conserved bacterial genome coding regions.By far,none of the archael group Ⅱ introns are reported in the cellular genes of their hosts.In living cells,some of the group Ⅱ introns were found to self-splice from their pre-RNAs following two consecutive transesterification reactions,then join the flanking exons and release the intron in a typical lariat form.The splicing reactions of many,not all group Ⅱ introns are assisted by proteins encoded by either the introns themselves,or other genes of the host organisms.Group Ⅱ introns can integrate efficiently into intronless alleles of the same gene or at much lower frequencies into ectopic sites.The mobility of group Ⅱ introns depends on the functions of intron encoded proteins(endonuclease and reverse transcriptase)and the RNA sequence and structure of the introns.In this article,we intend to give an overview about the recent research progress and insights of group Ⅱ introns.The diversity,origin and evolution of group Ⅱ introns will be primarily focused.