Control of polyadenylation and alternative splicing of transcripts from adjacent genes in a procyclin expression site: a dual role for polypyrimidine tracts in trypanosomes?

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Abstract
The procyclln-associated genes (PAGs) of Trypanosoma brucel are located downstream of tandemly repeated procyclin genes and belong to the same α-amanitln-resistant polycistronlc transcription units. In procycllc form trypanosomes the PAG 1 pre-mRNA is alternatively spliced to give rise to three transcripts of 2.7 kb, 1.8 kb and 1.3 kb. The two larger transcripts contain additional short open reading frames (ORFs) upstream of the major ORF. Trans-spliclng to generate these transcripts occurs downstream of three different polypyrimidine tracts. A minor population of procyclin mRNAs is also generated by alternative splicing at a polypyrimidine tract that begins 524 bp upstream of the major splice acceptor site of the procyclin β-gene. The same polypyrimidine tract is also required for accurate polyadenylation of mRNAs from the upstream procyclin α-gene (1). Alternatively polyadenylated forms of PAG 1 mRNAs can also be detected. All polyadenylation sites are found at a similar distance upstream of splice-acceptor sites, in each case with a polypyrimidine tract between them. Our results point to a dual role for polypyrimidine tracts in the maturation of trypanosome mRNAs.