The key difference between group I and group II introns is that in group I introns, the splicing reaction is initiated by a guanosine cofactor, while in group II introns, the splicing reaction is initiated by internal adenosine.
Pre-mRNA is the primary transcript that has both introns and exons. Pre-mRNA should convert into mRNA prior to translation. RNA splicing or pre-mRNA splicing is one such post-transcriptional modification. In RNA splicing, introns are removed from the pre-mRNA molecule, and exons are joined together. Group I and Group II introns are self-splicing introns. They splice off from pre-mRNA molecule without the help of any other enzyme. Therefore, they are RNA enzymes or ribozymes that catalyze their own splicing from pre-mRNA. Moreover, they have the ability to function as mobile elements. During the splicing, a series of trans-esterification reactions take place in order to excise the intron and ligate the exons. These ribozymes are present in all three domains, including bacteria, archaea and eukaryotes.
CONTENTS
1. Overview and Key Difference
2. What are Group I Introns
3. What are Group II Introns
4. Similarities Between Group I and Group II Introns
5. Side by Side Comparison – Group I vs Group II Introns in Tabular Form
6. Summary
What are Group I Introns?
Group I introns are a type of self-splicing ribozymes found in bacteria, bacteriophages and eukaryotes (organellar and nuclear genomes). They are found in essential genes. They are able to catalyze their own splicing from the pre-mRNA molecule. Group I introns may have a few hundred to three thousand nucleotides. Moreover, they exhibit little sequence similarity across organisms.
Figure 01: Group I Introns
The secondary structures are highly conserved in four short regions. There are two transesterification reaction steps of splicing. Group I introns initiate splicing mechanism by the nucleophilic attack of the 3’ hydroxyl of a Guanosine cofactor at the 5P splice site.
What are Group II Introns?
Group II introns are a type of self-splicing introns found in organisms belonging to all three domains. They are ribozymes that catalyze their own splicing reactions from pre-mRNA. They are found in rRNA, tRNA and protein-coding genes. But they are not found in nuclear genomes, unlike group I introns.
Figure 02: Group II Introns
Group II introns catalyze splicing via two transesterification steps similar to group I introns. These enzymes initiate the splicing reaction by the nucleophilic attack of 2′ OH of the branch site adenosine on the 5′ splice junction. During splicing reactions, group II introns form a lariat like structure. Moreover, intron splicing takes place in the absence of GTP.
What are the Similarities Between Group I and Group II Introns?
- Group I and group II introns are two types of RNA enzymes, ribozymes that catalyze their own splicing by different mechanisms.
- They are large ribozymes.
- Both are found in all three domains.
- They are mobile elements.
- Moreover, they are found in rRNA, tRNA and protein-coding genes.
- Both enzymes are using as tools in biotechnology and molecular medicine for targeted gene knock-out/knock-down, gene delivery or gene therapy systems.
What is the Difference Between Group I and Group II Introns?
Group I introns are ribozymes found in bacteria, bacteriophages and eukaryotic organellar and nuclear genomes. Group II introns are ribozymes found in bacteria, archaea, and eukaryotic organelles. Moreover, the group I introns initiate splicing reaction by the nucleophilic attack of the 3′ hydroxyl of a guanosine cofactor at the 5P splice site while group II introns initiate splicing reaction by the nucleophilic attack of the 2′ OH of the branch site adenosine on the 5′ splice junction. So, this is the key difference between group I and group II introns.
Furthermore, group II introns form a lariat like structure during the splicing while group I introns do not form. Thus, this is another significant difference between group I and group II introns. Besides, the group I introns are found in eukaryotic nuclear genomes while group II introns are not found in eukaryotic nuclear genomes.
Below infographic lists the differences between group I and group II introns in tabular form for side by side comparison.
Summary – Group I vs Group II Introns
Group I and II introns are large ribozymes that catalyze a transesterification reaction to splice out introns from the primary transcript. They are found in all three domains. They both are mobile genetic elements. Moreover, they are used as tools in biotechnology and molecular medicine. However, the group I introns initiate splicing reaction by the nucleophilic attack of the 3′ OH of a guanosine cofactor at the 5P splice site. But, the group II introns initiate splicing reaction by the nucleophilic attack of the 2′ OH of the branch site adenosine on the 5′ splice junction. Moreover, group II introns form a lariat like structure during the splicing while group I introns do not form a lariat like structure. Thus, this is the summary of the difference between group I and group II introns.
Reference:
1. Bonen, J. Vogel, et al. “Evolution of Group II Introns.” Mobile DNA, BioMed Central, 1 Jan. 1970, Available here.
2. Tourasse, Nicolas J, and Anne-Brit Kolstø. “Survey of Group I and Group II Introns in 29 Sequenced Genomes of the Bacillus Cereus Group: Insights into Their Spread and Evolution.” Nucleic Acids Research, Oxford University Press, Aug. 2008, Available here.
Image Courtesy:
1. “RF00028” By This image is taken from the Rfam database (Public Domain) via Commons Wikimedia
2. “IntronGroupII” By User:Sangak – Own work (Public Domain) via Commons Wikimedia