Difference between Group I and Group II introns

group I vs group II introns
group I vs group II introns

In many eukaryotic genes, the coding sequences are interrupted by blocks of non-coding sequences. These intervening sequences are termed introns. Exons can be defined as any region of the immature RNA that is retained in the mature RNA. By the process of RNA splicing, introns are removed from the pre-mRNA. Group I and Group II introns exhibit self-splicing mechanisms. We will further discuss the difference between Group I and Group II introns.

The chemistry of RNA Splicing

Before we dig deeper into the difference between Group I and Group II introns, let us first have a look at the chemistry of RNA splicing. The number of introns found in a gene varies significantly. They can be one in case of most intron-containing yeast genes to 363 in human Titin gene found in humans. Some specific nucleotide sequences mark the borders between intervening introns and coding exons. The intron-exon boundary at the 5′ end of the intron is called the 5′ splice site (donor site) and the 3′ end of the intron is called the 3′ splice site (acceptor site).  The consensus sequence at the 5′ splice site is GU while in the case of the 3′ splice site is AG. 

An intron is removed by two successive transesterification reactions. During the course of these reactions, existing phosphodiester linkages are broken and new ones are formed. The first reaction is initiated when the 2’OH of conserved A at the branch site in the intron attacks the phosphoryl group at the conserved G in the 5′ splice site. As a result of this nucleophilic attack, the phosphodiester bond between the sugar and the phosphate group at the 5′ end between the intron and exon is broken. The freed 5′ end of the intron gets joined to the A in the branch site. The leaving group after the first transesterification reaction is a 5′ exon. The 3’OH of the 5′ exon attacks the 3′ splice site. As a result of the second transesterification reaction, the intron leaves as a lariat. The 5′ and 3′ exons are joint.

The excised intron is immediately degraded after it has been removed and hence, it is not available for a  reverse reaction.

Self-Splicing introns

In the case of self-splicing, intron itself folds into a specific conformation and catalyses its own release. This splicing occurs in absence of any proteins or other RNA molecules.

Difference between Group I and Group II introns

group I introns vs group II introns

The group II intron splicing mechanism is very similar to the chemistry of spliceosome mediated splicing. The conserved A from the branch site of intron attacks the phosphodiester bond at the 5′ splice site and further, the 3’OH of the 5′ splice site attacks the 3′ splice site. The intron leaves as branched lariat and the 5′ and 3′ exons fuse.

group I vs group II introns

Source- Molecular Biology of the gene- James Watson

Where as, in the case of group I introns, RNA folds in a particular manner such that a guanine binding pocket is formed that allows the binding of a free guanine nucleotide. The 3’OH of the free guanine nucleotide acts as a nucleophile for the 5′ splice site. In the second transesterification reaction, the 3′ end of the 5′ exon attacks the 3′ splice site. As a result, the intron leaves as a linear structure and the 5′ and 3′ exons fuse.

Group I introns contain a specialised internal guide sequence that is responsible for base pairing with the 5′ splice site and hence, determines the particular site of nucleophilic attack by G nucleotide.

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