Many trypanosome messenger RNAs share a common 5′ terminal sequence

The mRNAs for different variant surface antigens of Trypanosoma brucei start with the same 35 nucleotides. This sequence is encoded by a separate mini-exon, located in a 1.35-kb repetitive element. We have reported that trypanosomes contain many transcripts that hybridize to mini-exon probes, even if they do not make the surface antigens. We show here that these transcripts have the mini-exon sequence at their 5' end; they do not contain other sequences from the mini-exon repeat element and are polyadenylated. We have cloned DNA complementary to trypanosome mRNAs and randomly selected 17 clones containing mini-exon sequences. Thirteen of these are derived from different genes that do not code for surface antigens. We conclude that the mini-exon sequence is a common element at the 5' end of many trypanosome mRNAs. As the 200 genes for mini-exons are highly clustered, linkage of the mini-exon sequence to the remainder of most mRNAs may require discontinuous transcription.