Group III intron is a class of

intron An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e., a region inside a gene."The notion of the cistron .e., gen ...

s found in mRNA genes of chloroplasts in

euglenid Euglenids or euglenoids are one of the best-known groups of eukaryotic flagellates: single-celled organisms with flagella, or whip-like tails. They are classified in the phylum Euglenophyta, class Euglenida or Euglenoidea. Euglenids are common ...

protist A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...

s. They have a conventional group II-type dVI with a bulged adenosine, a streamlined dI, no dII-dV, and a relaxed splice site consensus. Splicing is done with two

transesterification Transesterification is the process of exchanging the organic functional group R″ of an ester with the organic group R' of an alcohol. These reactions are often catalyzed by the addition of an acid or base catalyst. Strong acids catalyze the r ...

reactions with a dVI bulged adenosine as initiating

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

; the intron is excised as a lariat. Not much is known about how they work, although an isolated chloroplast transformation system has been constructed.

Discovery and identification

In 1984, Montandon and Stutz reported examples of a novel type of introns in Euglena

chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

. In 1989, David A. Christopher and Richard B. Hallick found a few more examples and proposed the name "Group III introns" to identify this new class with the following characteristics: * Group III introns are much shorter than other self-splicing intron classes, ranging from 95 to 110 nucleotides amongst those known to Christopher and Hallick, and identified in chloroplasts. On the other hand, Christopher and Hallick stated: "By contrast, the smallest Euglena

group II intron ... is 277 nucleotides." * Their conserved sequences proximal to the splicing sites have similarities to those of group II introns, but have fewer conserved positions. * They do not map into the conserved secondary structure of group II introns. (Indeed, Christopher and Hallick were unable to identify any conserved secondary structure elements among group III introns.) * They are usually associated with genes involved in translation and transcription. * They are very A+T rich. In 1994, discovery of a group III intron with a length of one order of magnitude longer indicated that length alone is not the determinant of splicing in Group III introns. Splicing of group III introns occurs through lariat and circular RNA formation. Similarities between group III and nuclear introns include conserved 5' boundary sequences, lariat formation, lack of internal structure, and ability to use alternate splice boundaries.

References

External links

GO:0000374
- Gene ontology entry for Group III intron splicing {{DEFAULTSORT:Group Iii Intron RNA Ribozymes RNA splicing

Discovery and identification

See also

References

External links