Trypanosoma brucei: a surface antigen mRNA is discontinuously transcribed from two distinct chromosomes

The mRNAs for variant surface glycoproteins (VSGs) and many other proteins in Trypanosoma brucei start with the same sequence of 35 nucleotides, encoded by a separate mini-exon. There are approximately 200 mini-exon genes per trypanosome and these are highly clustered on large chromosomes. We have found two trypanosome variants that express a VSG gene located on a small, 225-kb chromosome. Each gene yields a mRNA containing the 35-nucleotide sequence even though the 225-kb chromosome does not contain a complete mini-exon gene. These results provide a strong support for the hypothesis that transcription of protein-coding genes in trypanosomes is discontinuous.