Deadenylases specifically catalyze the degradation of eukaryotic mRNA poly(A) 
tail in the 3'- to 5'-end direction with the release of 5'-AMP as the product. 
Among the deadenylase family, poly(A)-specific ribonuclease (PARN) is unique in 
its domain composition, which contains three potential RNA-binding domains: the 
catalytic nuclease domain, the R3H domain and the RRM domain. In this research, 
we investigated the roles of these RNA-binding domains by comparing the 
structural features and enzymatic properties of mutants lacking either one or 
two of the three RNA-binding domains. The results showed that the R3H domain had 
the ability to bind various oligonucleotides at the micromolar level with no 
oligo(A) specificity. The removal of the R3H domain dissociated PARN into 
monomers, which still possessed the RNA-binding ability and catalytic functions. 
Unlike the critical role of the RRM domain in PARN processivity, the removal of 
the R3H domain did not affect the catalytic pattern of PARN. Our results 
suggested that both R3H and RRM domains were essential for the high affinity of 
long poly(A) substrate, but the R3H domain did not contribute to the substrate 
recognition of PARN. Compared to the RRM domain, the R3H domain played a more 
important role in the structural integrity of the dimeric PARN. The multiple 
RNA-binding domain architecture endows PARN the property of highly efficient 
catalysis in a highly processive mode.