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Group III intron

 

 
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Group III intron
 
 
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Group III intron is a class of intron
Intron

Introns, derived from the term "intragenic regions" and also called intervening sequence , are DNA regions in a gene that are not translated into proteins....
s found in mRNA genes of chloroplasts in euglenoid protist
Protist

Protists ; eukaryote microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy....
s. They have a conventional group II-type
Group II intron

Group II intron is a class of self-catalytic ribozymes and retroelements found in rRNA, tRNA, mRNA of organelles in fungi, plants, protists, and bacteria....
 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 alcohol group of an ester compound with another alcohol. These reactions are often catalyst by the addition of an acid or base ....
 reactions with a dVI bulged adenosine as initiating nucleophile
Nucleophile

In chemistry, a nucleophile is a reagent that forms a chemical bond to its reaction partner by donating both bonding electrons. Because nucleophiles donate electrons, they are by definition Lewis bases ....
; the intron is excised as a lariat.

Discovery and identification
Montandon,P. and Stutz,E.






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Group III intron is a class of intron
Intron

Introns, derived from the term "intragenic regions" and also called intervening sequence , are DNA regions in a gene that are not translated into proteins....
s found in mRNA genes of chloroplasts in euglenoid protist
Protist

Protists ; eukaryote microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy....
s. They have a conventional group II-type
Group II intron

Group II intron is a class of self-catalytic ribozymes and retroelements found in rRNA, tRNA, mRNA of organelles in fungi, plants, protists, and bacteria....
 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 alcohol group of an ester compound with another alcohol. These reactions are often catalyst by the addition of an acid or base ....
 reactions with a dVI bulged adenosine as initiating nucleophile
Nucleophile

In chemistry, a nucleophile is a reagent that forms a chemical bond to its reaction partner by donating both bonding electrons. Because nucleophiles donate electrons, they are by definition Lewis bases ....
; 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 chloroplast
Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryote organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve Thermodynamic free energy in the form of Adenosine triphosphate and reduce NADP to NADPH through a complex set of processes called photosynthesis....
. 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 chloroplast
    Chloroplast

    Chloroplasts are organelles found in plant cells and other eukaryote organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve Thermodynamic free energy in the form of Adenosine triphosphate and reduce NADP to NADPH through a complex set of processes called photosynthesis....
     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.