As part of their replication, viruses must pack newly made DNA to near-crystalline density in a small protein shell. But DNA is negatively charged, and repulsion of these charges creates a strong barrier to packing. Positively charged polyamines naturally available in cells help screen those interactions and accelerate packing, but there can be too much of a good thing: At higher concentrations, polyamines slow and stall packing, reports a team led by Douglas E. Smith of the University of California, San Diego (Phys. Rev. Lett. 2014, DOI: 10.1103/physrevlett.112.248101). Smith and colleagues used optical tweezers to study the effects of the polyamine spermidine3+, NH3(CH2)3NH2(CH2)4NH33+, on DNA packaging in a virus that infects bacteria. The virus uses a motor protein to reel DNA into a new virus shell. They found that 0.8 mM spermidine3+ reduced the packing forces, increasing the motor’s velocity and filling rate. At 5 mM spermidine3+, motor velocity sometimes increased further still, but more often the motor slowed and stalled, decreasing the packaging rate overall. The researchers suggest that higher concentrations of spermidine3+ induce attractive DNA conformations that could impede the motor.