Group III intron
Encyclopedia
Group III intron is a class of intron
s found in mRNA genes of chloroplasts in euglenoid protist
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 by two transesterification
reactions with a dVI bulged adenosine as initiating nucleophile
; the intron is excised as a lariat.
. In 1989, David A.Christopher and Richard B.Hallick proposed the title, Group III introns to identify this new class with the following characteristics:
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 (Copertino DW., Hall ET. et al. 1994)
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.
Intron
An intron is any nucleotide sequence within a gene that is removed by RNA splicing to generate the final mature RNA product of a gene. The term intron refers to both the DNA sequence within a gene, and the corresponding sequence in RNA transcripts. Sequences that are joined together in the final...
s found in mRNA genes of chloroplasts in euglenoid protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s. They have a conventional group II-type
Group II intron
Group II introns are a large class of self-catalytic ribozymes as well as mobile genetic element found within the genes of all three domains of life. Ribozyme activity can occur under high-salt conditions in vitro. However, assistance from proteins is required for in vivo splicing...
dVI with a bulged adenosine, a streamlined dI, no dII-dV, and a relaxed splice site consensus. Splicing is by two transesterification
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...
reactions with a dVI bulged adenosine as initiating nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
; the intron is excised as a lariat.
Discovery and identification
Montandon,P. and Stutz,E. (1984) and Hallick,R.B. et al. (1988 and 1989) reported examples of a novel type of introns in Euglena chloroplastChloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
. In 1989, David A.Christopher and Richard B.Hallick proposed the title, 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 chloroplastChloroplastChloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
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
RNA splicing
In molecular biology and genetics, splicing is a modification of an RNA after transcription, in which introns are removed and exons are joined. This is needed for the typical eukaryotic messenger RNA before it can be used to produce a correct protein through translation...
in Group III introns (Copertino DW., Hall ET. et al. 1994)
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.