THE EUGLENA-GRACILIS INTRON-ENCODED MAT2 LOCUS IS INTERRUPTED BY 3 ADDITIONAL GROUP-II INTRONS

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.