Trans-splicing and cis-splicing in the colourless Euglenoid, Entosiphon sulcatum.

The colourless Euglenoid Entosiphon sulcatum is thought to have diverged before the symbiotic event which gave rise to the photosynthetic Euglenoid species as Euglena gracilis. We have isolated genes encoding spliced leader-sequence RNA (SL-RNA) and we show that pre-mRNAs are matured via a trans-splicing reaction in E. sulcatum, as in the case of E. gracilis. The 2.5-kb repeated DNA fragment which encodes the SL-RNA gene also encodes a 5S rRNA gene as well as the genes for the small nuclear (sn) RNAs U1, U2 and U5. The presence of snRNA U1 indicates that the classical cis-splicing mechanism also exists in E. sulcatum. In addition, we show that the E. sulcatum beta-tubulin gene has the intron borders GU-AG, typical of spliceosome-matured introns which have not yet been found in E. gracilis. The probable origins of trans- and cis-mechanisms in Euglenoids are discussed.