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., gene ...
s found in mRNA genes of chloroplasts in
euglenid
Euglenids (euglenoids, or euglenophytes, formally Euglenida/Euglenoida, ICZN, or Euglenophyceae, ICBN) are one of the best-known groups of flagellates, which are excavate eukaryotes of the phylum Euglenophyta and their cell structure is typical ...
protist
A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...
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
In organic chemistry, transesterification is the process of exchanging the organic 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. The reaction ca ...
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 a ...
; the intron is excised as a lariat.[ Not much is known about how they work,]
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 known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it ...
.chloroplast
A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it ...
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.][ 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.][
]
See also
* Twintron
References
External links
GO:0000374
- Gene ontology
The Gene Ontology (GO) is a major bioinformatics initiative to unify the representation of gene and gene product attributes across all species. More specifically, the project aims to: 1) maintain and develop its controlled vocabulary of gene and g ...
entry for Group III intron splicing
{{DEFAULTSORT:Group Iii Intron
RNA
Ribozymes
RNA splicing