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Twintrons are introns-within-introns excised by sequential splicing reactions. Twintrons are presumably formed by the insertion of a mobile intron into an existing intron.

Twintron was discovered by Donald W. Copertino and Richard B. Hallick as a group II intron within another group II intron in Euglena chloroplast genome.[1] They found that splicing of both the internal and external introns occurs via lariat intermediates. Additionally, twintron splicing was found to proceed by a sequential pathway, the internal intron being removed prior to the excision of the external intron.

Since the original discovery, there have been other reports of Group III twintrons and GroupII/III twintrons in Euglena gracilis chloroplast. In 1993 a new type of complex twintron composed of four individual group III introns has been characterized.[2] The external intron was interrupted by an internal intron containing two additional introns. In 1995 scientists discovered the first non-Euglena twintron in cryptomonad alga Pyrenomonas salina.[3] In 2004, several twintrons were discovered in Drosophila.[4]

The majority of these twintrons have been characterized within the Euglena chloroplast genome but these elements have also been found in cryptomonad algae (Pyrenomonas salina),[5] and group I intron based twintrons (group I inserted within a group I intron) have been described in Didymium iridis.[6] Since the discovery of the psbF twintron, several categories of twintrons have been characterized. A twintron can be simple (external intron interrupted by 1 internal intron), or complex (external intron interrupted by multiple internal introns).[7] Most probably, the internal and external introns comprising the twintron element are from the same category; group I internal to group I,[8] group II internal to group II,[9] and group III internal to group III.[10] Mixed twintrons (consisting of introns belonging to different categories) were characterized from the Euglena gracilis rps3 gene in which an internal group II intron is found to interrupt an external group III intron.[11] In Rhodomonas salina (=Pyrenomonas salina) twintrons (nested group II/group III introns) were identified where the internal intron lost its splicing capacity, essentially merging with the outer intron forming one splicing unit.[12] Recently, two novel twintrons have been uncovered within the fungal mitochondrial genome, one at position mS917 of the Cryphonectria parasitica mt-rns gene, where a group ID intron encoding a LAGLIDADG ORF invaded another ORF-less group ID intron. Another twintron complex was detected at position mS1247 of the Chaetomium thermophilumhere mt-rns gene, a group IIA1 intron invaded the open reading frame embedded within a group IC2 intron.[13] The mS1247 twintron represents the first recorded fungal mitochondrial mixed twintron consisting of group II intron as an internal intron and a group I intron as an external intron. In mS1247 twintron, splicing of the internal group IIA1 intron reconstitutes the open reading frame encoded within the group IC2 intron and thus facilitates the expression of the encoded homing endonuclease. The mS1247 twintron encod ORF have been biochemically characterized and the results showed that it is an active homing endonuclease that could potentially mobilize the twintron to rns genes that have not yet been invaded by this mobile composite element.[14]


  1. ^ Copertino DW, Hallick RB (February 1991). "Group II twintron: an intron within an intron in a chloroplast cytochrome b-559 gene". The EMBO Journal. 10 (2): 433–42. PMC 452664Freely accessible. PMID 1899376. 
  2. ^ Drager RG, Hallick RB (May 1993). "A complex twintron is excised as four individual introns". Nucleic Acids Research. 21 (10): 2389–94. doi:10.1093/nar/21.10.2389. PMC 309537Freely accessible. PMID 7685079. 
  3. ^ Maier UG, Rensing SA, Igloi GL, Maerz M (January 1995). "Twintrons are not unique to the Euglena chloroplast genome: structure and evolution of a plastome cpn60 gene from a cryptomonad". Molecular & General Genetics. 246 (1): 128–31. doi:10.1007/BF00290141. PMID 7823908. 
  4. ^ Scamborova P, Wong A, Steitz JA (March 2004). "An intronic enhancer regulates splicing of the twintron of Drosophila melanogaster prospero pre-mRNA by two different spliceosomes". Molecular and Cellular Biology. 24 (5): 1855–69. doi:10.1128/MCB.24.5.1855-1869.2004. PMC 350559Freely accessible. PMID 14966268. 
  5. ^ Maier, UG; Rensing, SA; Igloi, GL; Maerz, M (6 January 1995). "Twintrons are not unique to the Euglena chloroplast genome: structure and evolution of a plastome cpn60 gene from a cryptomonad.". Molecular & general genetics : MGG. 246 (1): 128–31. doi:10.1007/bf00290141. PMID 7823908. 
  6. ^ Einvik, C; Nielsen, H; Westhof, E; Michel, F; Johansen, S (May 1998). "Group I-like ribozymes with a novel core organization perform obligate sequential hydrolytic cleavages at two processing sites.". RNA (New York, N.Y.). 4 (5): 530–41. doi:10.1017/s1355838298971758. PMC 1369637Freely accessible. PMID 9582095. 
  7. ^ Robart, AR; Zimmerly, S (2005). "Group II intron retroelements: function and diversity.". Cytogenetic and Genome Research. 110 (1–4): 589–97. doi:10.1159/000084992. PMID 16093712. 
  8. ^ Einvik, C; Elde, M; Johansen, S (17 September 1998). "Group I twintrons: genetic elements in myxomycete and schizopyrenid amoeboflagellate ribosomal DNAs.". Journal of biotechnology. 64 (1): 63–74. doi:10.1016/s0168-1656(98)00104-7. PMID 9823659. 
  9. ^ Copertino DW, Hallick RB (February 1991). "Group II twintron: an intron within an intron in a chloroplast cytochrome b-559 gene". The EMBO Journal. 10 (2): 433–42. PMC 452664Freely accessible. PMID 1899376. 
  10. ^ Copertino, DW; Hall, ET; Van Hook, FW; Jenkins, KP; Hallick, RB (25 March 1994). "A group III twintron encoding a maturase-like gene excises through lariat intermediates.". Nucleic Acids Research. 22 (6): 1029–36. doi:10.1093/nar/22.6.1029. PMC 307926Freely accessible. PMID 7512259. 
  11. ^ Copertino, DW; Hallick, RB (December 1993). "Group II and group III introns of twintrons: potential relationships with nuclear pre-mRNA introns.". Trends in Biochemical Sciences. 18 (12): 467–71. doi:10.1016/0968-0004(93)90008-b. PMID 8108859. 
  12. ^ Khan, H; Archibald, JM (May 2008). "Lateral transfer of introns in the cryptophyte plastid genome.". Nucleic Acids Research. 36 (9): 3043–53. doi:10.1093/nar/gkn095. PMC 2396441Freely accessible. PMID 18397952. 
  13. ^ Hafez, M; Majer, A; Sethuraman, J; Rudski, SM; Michel, F; Hausner, G (April 2013). "The mtDNA rns gene landscape in the Ophiostomatales and other fungal taxa: twintrons, introns, and intron-encoded proteins.". Fungal Genetics and Biology. 53: 71–83. doi:10.1016/j.fgb.2013.01.005. PMID 23403360. 
  14. ^ Guha, TK; Hausner, G (April 2014). "A homing endonuclease with a switch: characterization of a twintron encoded homing endonuclease.". Fungal Genetics and Biology. 65: 57–68. doi:10.1016/j.fgb.2014.01.004. PMID 24508098.