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Group III intron

Group III intron is a class of introns found in mRNA genes of chloroplasts in euglenoid protists. 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 exercised as a lariat.

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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. In 1989, David A.Christopherl and Richard B.Hallickl proposed the title, Group III introns to identify this new class with the following characteristics:

They are small and remarkably uniform in size, with a range of 95-110 nt, and an average size of 102 nt. By contrast, the smallest Euglena chloroplast group intron (rps8 intron 3) is 277 nt.
They have degenerate versions of the groupII intron consensus boundary sequences.
They lack the highly conserved secondary structural features characteristic of group II introns, a central core with 6 radiating, helical domains I-VI.
They are located primarily, but not exclusively, in genes for components of the Euglena chloroplast translation and transcription machinery.
The group III introns are very A/T rich, with a base bias of T > A > G > C.

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.