Accurate polyadenylation of procyclin mRNAs in Trypanosoma brucei is determined by pyrimidine-rich elements in the intergenic regions.

Polycistronic precursor RNAs from trypanosomes are processed into monocistronic mRNAs by the excision of intergenic sequences and the addition of a 39-nucleotide spliced leader by trans splicing. These mRNAs are also polyadenylated, yet they do not contain the hexamer AAUAAA within their 3' untranslated regions (UTRs). To identify the signals required for the accurate polyadenylation of mRNAs, we tested the effects of deletions in either the procyclin 3' UTR or the downstream intergenic region on the polyadenylation of transcripts from a reporter gene. Deletion of the entire 3' UTR does not affect polyadenylation, but a crucial element is located in the intergenic region and includes a pyrimidine-rich sequence from positions 79 to 112 followed by an AG dinucleotide. Related motifs are also found a similar distance downstream of other genes in both the procyclin and the variant surface glycoprotein expression sites. These sequences bear a strong resemblance to splice acceptor sites, but they are generally several hundred base pairs upstream of the major splice acceptor site of the next gene in the transcription unit. There is evidence, however, that some of them can give rise to alternatively spliced transcripts with unusually long 5' UTRs.