In molecular biology, atwintronis anintron-within-intron excised by sequentialsplicingreactions. A twintron is presumably formed by the insertion of a mobile intron into an existing intron.
Discovery
editTwintrons were discovered by Donald W. Copertino and Richard B. Hallick as agroup II intronwithin another group II intron inEuglenachloroplastgenome.[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 ofGroup IIItwintrons and GroupII/III twintrons in thechloroplastofEuglena gracilis.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-Euglenatwintron in cryptomonad algaPyrenomonas salina.[3]In 2004, several twintrons were discovered inDrosophila.[4]
Distribution
editThe majority of these twintrons have been characterized within theEuglenachloroplast 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 inDidymium 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 theEuglena gracilis rps3 gene in which an internal group II intron is found to interrupt an external group III intron.[11]InRhodomonas 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 formingonesplicing unit.[12]Recently, two novel twintrons have been uncovered within the fungal mitochondrial genome, one at position mS917 of theCryphonectria parasiticamt-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 theChaetomium thermophilumheremt-rns gene, a group IIA1 intron invaded theopen reading frameembedded 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]
See also
editReferences
edit- ^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.doi:10.1002/j.1460-2075.1991.tb07965.x.PMC452664.PMID1899376.
- ^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.PMC309537.PMID7685079.
- ^Maier UG, Rensing SA, Igloi GL, Maerz M (January 1995). "Twintrons are not unique to theEuglenachloroplast genome: structure and evolution of a plastome cpn60 gene from a cryptomonad ".Molecular & General Genetics.246(1): 128–31.doi:10.1007/BF00290141.PMID7823908.S2CID10545108.
- ^Scamborova P, Wong A, Steitz JA (March 2004)."An intronic enhancer regulates splicing of the twintron ofDrosophila melanogasterprospero pre-mRNA by two different spliceosomes ".Molecular and Cellular Biology.24(5): 1855–69.doi:10.1128/MCB.24.5.1855-1869.2004.PMC350559.PMID14966268.
- ^Maier, UG; Rensing, SA; Igloi, GL; Maerz, M (6 January 1995). "Twintrons are not unique to theEuglenachloroplast genome: structure and evolution of a plastome cpn60 gene from a cryptomonad ".Molecular & General Genetics.246(1): 128–31.doi:10.1007/bf00290141.PMID7823908.S2CID10545108.
- ^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.4(5): 530–41.doi:10.1017/s1355838298971758.PMC1369637.PMID9582095.
- ^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.PMID16093712.S2CID36481739.
- ^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.PMID9823659.
- ^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.doi:10.1002/j.1460-2075.1991.tb07965.x.PMC452664.PMID1899376.
- ^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.PMC307926.PMID7512259.
- ^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.PMID8108859.
- ^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.PMC2396441.PMID18397952.
- ^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.PMID23403360.
- ^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.PMID24508098.