Split genes in fungal mitochondria
Split genes are also found in mitochondria. Introns of these split genes in fungal mitochondria are classified into group I and group II introns on the basis of their internal organization. (i) Group I introns, which are found in majority of the fungal mitochondrial split genes, do not carry any conserved sequences at intron-exon junctions, but carry internally a short conserved sequence called 'internal guide sequence'
By internal pairing, this guide sequence brings the two intron-exon junctions together and help in splicing out the introns (see Expression of Gene : Protein Synthesis 3. RNA Processing (RNA Splicing, RNA Editing and Ribozymes)
for details). These group I introns are also found in the nuclear genes coding for rRNA in Tctrahymena
(a ciliate) and Physarutn
(a slime mold). Features similar to those of group I introns are also found in introns of phage T4 genes, (ii) Group II introns resemble nuclear genes and have consensus sequences (GT and APy) and a branch sequence that resembles the TACTAAC box. These introns are excised as lariats (see Expression of Gene : Protein Synthesis 3. RNA Processing (RNA Splicing, RNA Editing and Ribozymes)