Group III intron

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]

Discovery and identification

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]

• 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 group II intron ... is 277 nucleotides."^[5]
• 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.^[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]

See also

• Twintron