| Group I catalytic intron | |
|---|---|
 Predictedsecondary structure andsequence conservation of Group I catalytic intron | |
| Identifiers | |
| Symbol | Intron_gpI |
| Rfam | RF00028 |
| Other data | |
| RNA type | Intron |
| Domain(s) | Eukaryota;Bacteria;Viruses |
| GO | GO:0000372 |
| SO | SO:0000587 |
| PDB structures | PDBe |
Group I introns are large self-splicingribozymes. Theycatalyze their own excision frommRNA,tRNA andrRNA precursors in a wide range of organisms.[1][2][3][4] The coresecondary structure consists of nine paired regions (P1-P9).[5] These fold to essentially twodomains – the P4-P6 domain (the "scaffolding" domain, which is formed from the stacking of P5, P4, P6 and P6a helices) and the P3-P9 domain (the "catalytic" domain, which is formed from the P8, P3, P7 and P9 helices).[2] The secondary structure mark-up for this family represents only this conserved core. Certain subclasses of Group I introns also contain peripheral structural domains that provide additional structural support to the intron.[4][6] Group Iintrons often have longopen reading frames inserted inloop regions.
Splicing ofgroup I introns is processed by two sequentialtransesterification reactions.[3] First anexogenousguanosine or guanosinenucleotide (exoG) docks onto the active G-binding site located in P7, and then its 3'-OH is aligned to attack thephosphodiester bond at the "upstream" (closer to the 5' end) splice site located in P1,[6] resulting in a free 3'-OH group at the upstreamexon and the exoG being attached to the 5' end of the intron. Then the terminal G (omega G) of the intron swaps out the exoG and occupies the G-binding site, preparing the second ester-transfer reaction: the 3'-OH group of the upstream exon in P1 is aligned to attack the downstream splice site in P10, leading to theligation of the adjacent upstream and downstream exons and release of the catalytic intron.
The two-metal-ion mechanism seen in proteinpolymerases andphosphatases was proposed to be used by group I andgroup II introns to process the phosphoryl transfer reactions,[7] which was unambiguously proved by a high-resolution structure of theAzoarcus group I intron in 2006.[8]
Since the early 1990s, scientists started to study how the group I intron achieves its native structurein vitro, and some mechanisms of RNAfolding have been appreciated thus far.[12] It is agreed that thetertiary structure is folded after the formation of the secondary structure. During folding, RNA molecules are rapidly populated into different folding intermediates, the intermediates containing native interactions are further folded into the native structure through a fast folding pathway, while those containing non-native interactions are trapped inmetastable or stable non-native conformations, and the process of conversion to the native structure occurs very slowly. It is evident that group I introns differing in the set of peripheral elements display different potentials in entering the fast folding pathway. Meanwhile, cooperative assembly of the tertiary structure is important for the folding of the native structure. Nevertheless, folding of group I introns in vitro encounters boththermodynamic andkinetic challenges. A few RNA binding proteins andchaperones have been shown to promote the folding of group I introns in vitro and in bacteria by stabilizing the native intermediates, and by destabilizing the non-native structures, respectively.
Group I introns are distributed in bacteria, lowereukaryotes and higherplants. However, their occurrence in bacteria seems to be more sporadic than in lowereukaryotes, and they have become prevalent in higher plants. Thegenes that group Iintrons interrupt differ significantly: They interruptrRNA,mRNA andtRNAgenes in bacterial genomes, as well as inmitochondrial andchloroplastgenomes of lower eukaryotes, but only invade rRNA genes in thenuclear genome oflower eukaryotes. In higher plants, these introns seem to be restricted to a fewtRNA and mRNA genes of the chloroplasts and mitochondria.
Group I introns are also found inserted into genes of a wide variety ofbacteriophages ofGram-positive bacteria.[13] However, their distribution in the phage ofGram-negative bacteria is mainly limited to theT4, T-even andT7-like bacteriophages.[13][14][15][16]
Both intron-early and intron-late theories have found evidences in explaining the origin of group I introns.Some group I introns encodehoming endonuclease (HEG), which catalyzes intron mobility. It is proposed that HEGs move theintron from one location to another, from one organism to another and thus account for thewide spreading of the selfish group I introns. No biological role has beenidentified for group I introns thus far except for splicing of themselves from the precursorto prevent the death of the host that they live by. A small number of group I introns arealso found to encode a class of proteins called maturases that facilitate theintron splicing.