ATP serves as a nucleotide switch coupling the genome maturation and packaging motor complexes of a virus assembly machine.

The assembly of double-stranded DNA viruses, from phages to herpesviruses, is strongly conserved. Terminase enzymes processively excise and package monomeric genomes from a concatemeric DNA substrate. The enzymes cycle between a stable maturation complex that introduces site-specific nicks into the duplex and a dynamic motor complex that rapidly translocates DNA into a procapsid shell, fueled by ATP hydrolysis. These tightly coupled reactions are catalyzed by terminase assembled into two functionally distinct nucleoprotein complexes; the maturation complex and the packaging motor complex, respectively. We describe the effects of nucleotides on the assembly of a catalytically competent maturation complex on viral DNA, their effect on maturation complex stability and their requirement for the transition to active packaging motor complex. ATP plays a major role in regulating all of these activities and may serve as a 'nucleotide switch' that mediates transitions between the two complexes during processive genome packaging. These biological processes are recapitulated in all of the dsDNA viruses that package monomeric genomes from concatemeric DNA substrates and the nucleotide switch mechanism may have broad biological implications with respect to virus assembly mechanisms.