HIERARCHIES OF RNA-PROCESSING SIGNALS IN A TRYPANOSOME SURFACE-ANTIGEN MESSENGER-RNA PRECURSOR

Nearly all trypanosome mRNAs are synthesized as polycistronic precurso rs, from which mature mRNAs are excised by trans splicing and polyaden ylation. Polyadenylation of a procyclic acidic repetitive protein (PAR P, or procyclin) transcript was studied by transient transfection of c onstructs bearing a chloramphenicol acetyltransferase gene linked to t he PARP intergenic region. Polyadenylation usually occurred at A resid ues, about 100 bases upstream of a trans-splicing acceptor signal. The wild-type polyadenylation site has a cryptic trans-splicing signal ab out 100 bp downstream: deletion or inversion of this signal results in polyadenylation at multiple sites, upstream of other cryptic trans-sp licing signals. The PARP mRNA precursor appears to contain a hierarchy of possible processing signals, the function of cryptic ones being re vealed only when the dominant ones are deleted or moved. Correct polya denylation can be restored by addition of trans-splicing signals from other loci. The results indicate that polyadenylation is coupled to do wnstream trans splicing but that the products of the trans-splicing re action are not necessarily functional mRNAs.