We have recently shown that, in Euglena gracitis, leader sequences are transferred by trans-splicing to the vast majority of cytoplasmic mRNAs. Trans-splieing is involved in the maturation of the rbcS transcript, which encodes eight small subunits of the ribulose 1,5 bisphosphate carboxylase/ oxygenase. In this report, we show that the Eugh, mt rbcSgene introns are different from introns found in plant rbcS genes, In additiot~ these introns do not have the conserved 5' and 3' border sequences found in introns of euearyotie nuclear-encoded pre.mRNAs, and they do not present any homology with sell:splicing introns of groups I and II. Secondary structure analyses show that the 5' and 3' ends of Eagle/in introns can base-pair, suggesting that an Ulmsaal splicing mechanism exists in Euglena.

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