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The Euglena gracilis intron-encoded mat2 locus is interrupted by three additional group II introns.

The 4,144 nt Euglena gracilis chloroplast psbC intron 2 has been characterized as a single, cis-spliced 593 nt group II intron interrupted by an open reading frame of 758 codons in the loop region of domain IV. The 2,277 nt coding region of orf 758 is interrupted by two additional group II introns of 369 nt and 352 nt. Another 553 nt group II intron is located in the 5' untranslated leader region of orf 758. Because the psbC intron 2 orf encodes a maturase-like protein that has reverse transcriptase domains and a domain X characteristic of group II intron-encoded proteins, the locus has been designated mat2. The psbC intron 2 is the first member of a new category of twintron, characterized by introns within a gene within another intron. A potential role of psbC intron 2 as a "founder" intron involved in the spread of introns to new sites in the plastid genome of the Euglenophycae is discussed.