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]