The 4,144 nt Euglena gracilis chloroplast psbC intron 2 has been chara
cterized as a single, cis-spliced 593 nt group II intron interrupted b
y 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 additiona
l group II introns of 369 nt and 352 nt. Another 553 nt group II intro
n 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 intro
n-encoded proteins, the locus has been designated mat2. The psbC intro
n 2 is the first member of a new category of twintron, characterized b
y introns within a gene within another intron. A potential role of psb
C intron 2 as a ''founder'' intron involved in the spread of introns t
o new sites in the plastid genome of the Euglenophycae is discussed.