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]

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References
  1. Copertino DW, Hallick RB (December 1993). "Group II and group III introns of twintrons: potential relationships with nuclear pre-mRNA introns". Trends Biochem. Sci. 18 (12): 467–71. doi:10.1016/0968-0004(93)90008-b. PMID 8108859.
  2. Donald W. Copertino DW; Hall ET; Van Hook FW; Jenkins KP; 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.
  3. Doetsch, NA; Favreau, MR; Kuscuoglu, N; Thompson, MD; Hallick, RB (February 2001). "Chloroplast transformation in Euglena gracilis: splicing of a group III twintron transcribed from a transgenic psbK operon". Current genetics. 39 (1): 49–60. doi:10.1007/s002940000174. PMID 11318107.
  4. 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.
  5. 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.

See also
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