The <i>Euglena gracilis rbcS</i> gene contains introns with unusual borders

We have recently shown that, in Euglena gracilis, leader sequences are transferred by trans-splicing to the vast majority of cytoplasmic mRNAs. Trans-splicing 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 Euglena rbcS gene introns are different from introns found in plant rbcS genes. In addition these introns do not have the conserved 5' and 3' border sequences found in introns of eucaryotic nuclear-encoded pre-mRNAs, and they do not present any homology with self-splicing introns of groups I and II. Secondary structure analyses show that the 5' and 3' ends of Euglena introns can base-pair, suggesting that an unusual splicing mechanism exists in Euglena.