Question

What is the main support for the proposal that spliceosome-catalyzed pre-mRNA splicing probably evolved from the self-splicing intron mechanism?

Answer 1

ANSWER Self splicing intron are known as group II splicing system and are present in bacterial genome , mitochondria and chloroplast genome of eukaryotes. It is present in low number in bacterial genome. The endosymbiotic theory lead to origin of eukaryotes act as main factor which indicated the Spliceosome based splicing had been involved from self splicing. The presence of self splicing in mitochondria and chloroplast which themselves had been originated from Aerobic and photosynthetic bacteria respectively.  Group II intronic RNA sequences show very complex and conserved secondary structures that span 400–800 nucleotides. Group II intron sequences are organized in six main domains (DI–DVI) consisting of various functional “loops” and “bulges,” which are the basis of the ribozymal activity. The similarity in spliceosome based splicing and self splicing include common : two transesterification reaction and excision of lariat form intron.

The early mitochondria which is a descendent of Proteobacteria contain self splicing system. With time the intron of self splicing proliferated to the host level genome but after that self splicing activity decrease due to self splicing intron degeeration which was associated with the increase of trans-encoded RNAs and proteins that promoted efficient intron splicing, setting the basis of the proto spliceosomal machinery, and further releasing selective pressure on cis-intronic splicing elements. this laid to foundation and orginate of spliceosomal machinery for splicing.