ALTERNATIVE SPLICING WITHIN AND BETWEEN ALLELES OF THE ATPASE GENE 1 LOCUS OF TRYPANOSOMA-BRUCEI

The P-type ATPase gene TBA1 of Trypanosoma brucei belongs to a polycis tronic transcription unit. We anlayzed the structure and expression of a 4-kb region located immediately downstream from TBA1. This region i s unique and contains two large open reading frames transcribed into s table mRNAs. These putative genes, termed ADG1 and ADG2, can respectiv ely encode a 24-kDa and a 81-kDa protein. The intergenic spacings betw een the polyadenylation sites and the next 3' splice acceptor sites ar e very short: 148 bp between TBA1 and ADG1, and 127 bp between ADG1 an d ADG2. Transcripts from each of the two ADG1 alleles can be detected, indicating that both homologs are transcribed. These transcripts are differentially spliced due to a single base difference which destroys in one homolog the AG acceptor site present in the other. In the 'muta nt' allele an alternative downstream splice acceptor site is used. Des pite its sequence conservation in both alleles, this splice site is on ly used in the allele lacking the upstream AG acceptor site. The major population of ADG1 transcripts exhibit a long 5'-untranslated extensi on and no 3'-terminal tail, but a minor population shows a smaller 5'- untranslated region due alternative splicing closer to the initiation codon of the gene. The steady-state amounts of transcripts from indivi dual genes in this region are differentially stage-regulated.