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.