The stability of mRNAs is an important point in the regulation of gene 
expression in eukaryotes. The mRNA turnover pathways have been identified in 
yeast and mammals. However, mRNA turnover pathways in trypanosomes have not been 
widely studied. Deadenylation is the first step in the major mRNA turnover 
pathways of yeast and mammals. To better understand mRNA degradation processes 
in these organisms, we have developed an in vitro mRNA turnover system that is 
functional for deadenylation. In this system, addition of poly(A) homopolymer 
activates the deadenylation of poly(A) tails. The trypanosomal deadenylase 
activity is a 3'-->5' exonuclease specific for adenylate residues, generates 
5'-AMP as a product, is magnesium dependent, and is inhibited by neomycin B 
sulfate. These characteristics suggest similarity with other eukaryotic 
deadenylases. Furthermore, this activity is cap independent, indicating a 
potential difference between the trypanosomal activity and PARN, but suggesting 
similarity to Ccr4p/Pop2p activities. Extracts immunodepleted of Pab1p required 
the addition of poly(A) competition to activate deadenylation. Trypanosomal 
Pab1p functions as an inhibitor of the activity under in vitro conditions. Pab1p 
appears to be one of several mRNA stability proteins in trypanosomal extracts.