PolyA polymerase (PAP) adds a polyA tail onto the 3'-end of RNAs without a 
nucleic acid template, using adenosine-5'-triphosphate (ATP) as a substrate. The 
mechanism for the substrate selection by eubacterial PAP remains obscure. 
Structural and biochemical studies of Escherichia coli PAP (EcPAP) revealed that 
the shape and size of the nucleobase-interacting pocket of EcPAP are maintained 
by an intra-molecular hydrogen-network, making it suitable for the accommodation 
of only ATP, using a single amino acid, Arg(197). The pocket structure is 
sustained by interactions between the catalytic domain and the RNA-binding 
domain. EcPAP has a flexible basic C-terminal region that contributes to optimal 
RNA translocation for processive adenosine 5'-monophosphate (AMP) incorporations 
onto the 3'-end of RNAs. A comparison of the EcPAP structure with those of other 
template-independent RNA polymerases suggests that structural changes of 
domain(s) outside the conserved catalytic core domain altered the substrate 
specificities of the template-independent RNA polymerases.