RNA splicing
Fig. 33.3. Self splicing of group I and-group II introns, by two transesterification reactions.
Fig. 33.4. Common secondary structure of a group I intron in Tetrahymena, facilitated by internal pairing due to consensus sequences (P, Q and R, S) and the pairing at the ends due to internal guide sequence (IGS).
Self-splicing of group I introns
Self-splicing of RNA molecules involving group I introns, found in rRNA genes of Tetrahymena and Physarum nuclei, in fungal mitochondria and in phage T4, takes place through two transesterification reactions (exchange of phosphate esters, which leave the total number of phosphodiester bonds unchanged). Group I introns are characterized by (i) the absence of conserved sequences at splicing junction, and by (i) presence of short conserved consensus sequences internally. In the first transesterification, the 5' splice site is cleaved, as guanosine is added to the 5' end of intervening sequence (IVS) to be cleaved. In the second transesterification step, the 3' splice site is cleaved as the exons are joined (Fig. 33.3a). The excised IVS or intron can form a circle by cyclization reaction and these circles can again regenerate linear molecules due to autocatalysis. It means that excised IVS RNA and truncated versions of the IVS RNA also act as ribozymes to cleave, join or de-phosphorylate RNA substrates.
Fig. 33.3. Self splicing of group I and-group II introns, by two transesterification reactions.
Fig. 33.4. Common secondary structure of a group I intron in Tetrahymena, facilitated by internal pairing due to consensus sequences (P, Q and R, S) and the pairing at the ends due to internal guide sequence (IGS).
Fig. 33.5. A hammerhead secondary structure in viroids and virusoids showing site for self cleavage by arrow.
Fig. 33.3. Self splicing of group I and-group II introns, by two transesterification reactions.
Mitochondrial group II introns resemble nuclear introns and are excised as lariats like nuclear introns. They have consensus sequences at the splicing junctions, GT and APy and a branch sequence resembling TACTAAC box. The splicing reaction is autonomous unlike that in the nuclear introns, where trans-acting snRNPs take part in the formation of spliceosomes. Splicing in these group II introns gives rise to lariat directly, which then gives rise to a-linear form having no activity. This self splicing reaction may be regarded as an intermediate step between RNA mediated self splicing in group I introns and protein dependent RNA splicing of nuclear introns (Fig. 33.3 a).
Self splicing through formation of hammer-head
Incase of viroids (naked infectious RNA molecules capable of replicating) and virusoids = satellite RNA (encapsidated with plant viruses, and incapable of replicating independently), a consensus sequence forms a 'hammerhead' secondary structure, which helps in self cleavage (Fig. 33.5).