Group I catalytic intron

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Group I catalytic intron
GO
GO:0000372
SOSO:0000587
PDB structuresPDBe

Group I introns are large self-splicing

introns often have long open reading frames inserted in loop regions
.

Catalysis

exogenous guanosine or guanosine nucleotide (exoG) first docks onto the active G-binding site located in P7, and its 3'-OH is aligned to attack the phosphodiester bond at the 5' splice site located in P1, resulting in a free 3'-OH group at the upstream exon and the exoG being attached to the 5' end of the intron. Then the terminal G (omega G) of the intron swaps the exoG and occupies the G-binding site to organize the second ester-transfer reaction: the 3'-OH group of the upstream exon in P1 is aligned to attack the 3' splice site in P10, leading to the ligation
of the adjacent upstream and downstream exons and release of the catalytic intron.

Two-metal-ion mechanism seen in protein

phosphatases was proposed to be used by group I and group II introns to process the phosphoryl transfer reactions,[5] which was unambiguously proven by a high-resolution structure of the Azoarcus group I intron in 2006.[6]

A 3D representation of the Group I catalytic intron. This view shows the active site in the crystal structure of the Tetrahymena ribozyme.[7]
A 3D representation of the Group I catalytic intron. This is the crystal structure of a phage Twort group I ribozyme-product complex.[8]
A 3D representation of the Group I catalytic intron. This is the structure of the Tetrahymena ribozyme with a base triple sandwich and metal ion at the active site.[9]

Intron folding

Since the early 1990s, scientists started to study how the group I intron achieves its native structure

thermodynamic and kinetic challenges. A few RNA binding proteins and chaperones
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.

Distribution, phylogeny and mobility

Group I introns are distributed in bacteria, lower eukaryotes and higher plants. However, their occurrence in bacteria seems to be more sporadic than in lower eukaryotes, and they have become prevalent in higher plants. The genes that group I introns interrupt differ significantly: They interrupt

tRNA
genes in bacterial genomes, as well as in
mitochondrial and chloroplast
genomes of lower eukaryotes, but only invade rRNA genes in the nuclear genome of lower eukaryotes. In higher plants, these introns seem to be restricted to a few tRNA and mRNA genes of the chloroplasts and mitochondria.

Group I introns are also found inserted into genes of a wide variety of

T4, T-even and T7-like bacteriophages.[11][12][13][14]

Both intron-early and intron-late theories have found evidences in explaining the origin of group I introns. Some group I introns encode homing endonuclease (HEG), which catalyzes intron mobility. It is proposed that HEGs move the intron from one location to another, from one organism to another and thus account for the wide spreading of the selfish group I introns. No biological role has been identified for group I introns thus far except for splicing of themselves from the precursor to prevent the death of the host that they live by. A small number of group I introns are also found to encode a class of proteins called maturases that facilitate the intron splicing.

See also

References

  1. PMID 19667762
    . Retrieved 2010-07-15.
  2. ^
    S2CID 38185676
    .
  3. ^
    PMID 2197983
    .
  4. PMID 15922592
    .
  5. PMID 8341661
    .
  6. PMID 16697179
    .
  7. PMID 9841391
    .
  8. S2CID 33369317
    .
  9. PMID 15525509
    .
  10. PMID 9241415
    .
  11. ^
    PMID 10986228
    .
  12. PMID 15026408
    .
  13. PMID 15547290
    .
  14. PMID 19854925
    .

Further reading

External links
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