Figure 8

Measurement of nucleoid densities in Pithovirus and Mimivirus particles. (A) A bright field image without energy-filtering (cryo-EM at 200 kV). The small hexagonal and large oval shapes correspond to Mimivirus and Pithovirus particles, respectively. (B) Calculated EELS log-ratio image (t/λ, see Methods). Red and blue rectangles highlight regions for estimating nucleoid density profiles in Pithovirus and Mimivirus particles in (C). Black open boxes on the particles show the area used to statistically calculate the nucleoid density in (D). (C) Relative density line profiles (λ⁻¹, see Methods) of Pithovirus and Mimivirus particles calculated from measurements in the red and blue bands in (B). The profile shows that the values of the central areas (a–b and c–d) on each viral particle reach a plateau, and the calculated density of Pithovirus was relatively lower than that of Mimivirus. (D) Statistics of the estimated relative nucleoid density of Pithovirus and Mimivirus particles. The relative nucleoid density (λ⁻¹, see Methods) was calculated with each particle size (the particle width in Pithovirus; the capsid diameter in Mimivirus) in a central area of 150 nm × 150 nm and averaged in 9 Pithovirus and 36 Mimivirus particles. The nucleoid density of Pithovirus particle was estimated to be ~0.77 times the density of Mimivirus particles.