Group III intron is a class of introns found in mRNA genes of chloroplasts in euglenid 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. [1]: fig. 2 Splicing is done with two transesterification reactions with a dVI bulged adenosine as initiating nucleophile; the intron is excised as a lariat. [2] Not much is known about how they work, [1] although an isolated chloroplast transformation system has been constructed. [3]
In 1984, Montandon and Stutz reported examples of a novel type of introns in Euglena chloroplast. [4] 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: [5]
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. [2]
Splicing of group III introns occurs through lariat and circular RNA formation. [2] 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. [1]