CONTROL OF POLYADENYLATION AND ALTERNATIVE SPLICING OF TRANSCRIPTS FROM ADJACENT GENES IN A PROCYCLIN EXPRESSION SITE - A DUAL ROLE FOR POLYPYRIMIDINE TRACTS IN TRYPANOSOMES

The procyclin-associated genes (PAGs) of Trypanosoma brucei are locate d downstream of tandemly repeated procyclin genes and belong to the sa me cr-amanitin-resistant polycistronic transcription units. In procycl ic form trypanosomes the FAG 1 pre-mRNA is alternatively spliced to gi ve rise to three transcripts of 2.7 kb, 1.8 kb and 1.3 kb. The two lar ger transcripts contain additional short open reading frames (ORFs) up stream of the major ORF. Transsplicing to generate these transcripts o ccurs downstream of three different polypyrimidine tracts. A minor pop ulation of procyclin mRNAs is also generated by alternative splicing a t a polypyrimidine tract that begins 524 bp upstream of the major spli ce acceptor site of the procyclin beta-gene. The same polypyrimidine t ract is also required for accurate polyadenylation of mRNAs from the u pstream procyclin alpha-gene (1). Alternatively polyadenylated forms o f FAG 1 mRNAs can also be detected. All polyadenylation sites are foun d at a similar distance upstream of splice-acceptor sites, in each cas e with a polypyrimidine tract between them. Our results point to a dua l role for polypyrimidine tracts in the maturation of trypanosome mRNA s.