Cryo-EM and Negative Stain Specimen Preparation

The cryo-EM specimens were prepared as described previously39 (link). In
brief, an aliquot (∼4 μl) of DNA-nanogold sample at a
concentration of
∼20 μg ml⁻¹was placed on a glow-discharged lacey carbon film-coated copper grid (Cu-200LC,
Pacific Grid-Tech, San Francisco, CA, USA). The samples were blotted with filter
paper from both sides at ∼90% humidity and
4 °C with a Leica EM GP rapid-plunging device (Leica, Buffalo
Grove, IL, USA) and then flash-frozen in liquid ethane. The flash-frozen grids
were transferred into liquid nitrogen for storage. The NS specimens were
prepared by OpNS protocol as described previously19 (link)22 (link). In
brief, an aliquot (∼4 μl) of DNA-nanogold sample at a
concentration of
∼20 μg ml⁻¹was placed on a thin carbon-coated 200-mesh copper grid (Cu-200CN, Pacific
Grid-Tech; CF200-Cu, Electron Microscopy Sciences, Hatfield, PA, USA) that had
been glow-discharged. After ∼1 min of incubation, the excess
solution was blotted with filter paper. The grid was then washed with water and
stained with 1% (w/v) uranyl formate on Parafilm before air-drying
with nitrogen19 (link)22 (link).