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

The colourless Euglenoid Entosiphon sulcatum is thought to have diverged be fore the symbiotic event which gave rise to the photosynthetic Euglenoid sp ecies as Euglena gracilis. We have isolated genes encoding spliced leader-s equence RNA (SL-RNA) and we show that pre-mRNAs are matured via a trans-spl icing reaction in E. sulcatum, as in the case of E. gracilis. The 2.5-kb re peated DNA fragment which encodes the SL-RNA gene also encodes a 5S rRNA ge ne 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 al so exists in E. sulcatum. In addition, we show that the E. sulcatum beta-tu bulin pnp has thp intron borders GU-AG, typical of spliceosome-matured intr ons which have not yet been found in E. gracilis. The probable origins of t rans- and cis-mechanisms in Euglenoids an discussed.