IN EUGLENA, SPLICED-LEADER RNA (SL-RNA) AND 5S RIBOSOMAL-RNA GENES ARE TANDEMLY REPEATED

In Euglena gracilis, a 26 nucleotide leader sequence (spliced leader sequence = SL) is transferred by trans-splicing to the 5' end of a vast majority of cytoplasmic mRNAs (8). The SL originates from the 5' extremity of a family of closely related snRNAs (SL-RNAs) which are about 100 nucleotide long. In this paper we present the nucleotide sequences of two SL-RNA genes, confirming the sequences previously established by sequencing purified SL-RNAs. Although some SL-RNA genes are dispersed throughout the genome, we show that the majority of SL-RNA genes are located on 0.6 kb repeated units which also encode the cytoplasmic 5S rRNA. We estimate that the copy number of these repeated units is about 300 per haploid genome. The association of SL-RNA and 5S rRNA genes in tandemly repeated units is also found in nematodes but paradoxically does not exist in trypanosomes which are phylogenically much closer to Euglena. We also show that a high number of sequences analogous to the 26 nucleotide SL are dispersed throughout the genome and are not associated with SL-RNAs.