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 excised as a lariat.

Discovery and identification

Montandon,P. and Stutz,E. (1984) ^[1] and Hallick,R.B. et al. (1988 and 1989) reported examples of a novel type of introns in Euglena chloroplast. In 1989, David A.Christopher and Richard B.Hallick proposed the title, Group III introns to identify this new class with the following characteristics:^[2]

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."
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 (Copertino DW., Hall ET. et al. 1994^[3])

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.

References

^ Montandon PE, Stutz E (September 1983). "Nucleotide sequence of a Euglena gracilis chloroplast genome region coding for the elongation factor Tu; evidence for a spliced mRNA". Nucleic Acids Res. 11 (17): 5877–92. doi:10.1093/nar/11.17.5877. PMC 326324. PMID 6310519. http://nar.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=6310519.
^ Christopher DA, Hallick RB (October 1989). "Euglena gracilis chloroplast ribosomal protein operon: a new chloroplast gene for ribosomal protein L5 and description of a novel organelle intron category designated group III". Nucleic Acids Res. 17 (19): 7591–608. doi:10.1093/nar/17.19.7591. PMC 334869. PMID 2477800. http://nar.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=2477800.
^ Donald W. Copertino DW, Hall ET, Van Hook FW, Jenkins KP and Hallick RB (1994). "A group III twintron encoding a maturase-like gene excises through lariat intermediates.". Nucleic Acids Res. 22 (6): 1029–36. doi:10.1093/nar/22.6.1029. PMC 307926. PMID 7512259. http://nar.oxfordjournals.org/cgi/reprint/22/6/1029.