The overlaid catalytic center of VP55 in the VP55 monomer and the VP39–VP55 heterodimer based on a superposition of the central domain (residues 161–278). The residues (as well as 3′-dATP and the RNA primer) from the 3′-dATP/RNA-bound heterodimer (PDB entry 3erc
, chain C) are colored gray, while those from chain B in the VP55 monomer are green. The 2Fo − Fc OMIT map for the VP55 monomer is shown at the 1σ contour level. Residues discussed in the text are labeled.