Figure 4

Cryo-electron micrographs showing fields of (A) Head II at pH 7.5; (B) Head II at pH 7.5 after heating at 85°C; after such heating, Head II had a tendency to form close-packed arrays in vitrified films, as here, but dispersed areas were also observed. However, there is no evident change in size and shape. Such variability in appearance as is observed reflects differences in viewing geometry, for example, the hexagonal particles are viewed along a three-fold axis of symmetry or close to it. (C, D) Comparison of fields of K169Y capsids prepared by acidifying aliquots of a Prohead II isolate to pH 3.9 ((C) essentially all EI-II-like capsids) and pH 4.3 ((D) an approximately equal mixture of EI-II and balloons). In (D), a few examples of EI-II capsids are marked with white arrows, and those of balloons with black arrows. (E, F) Comparison of Head I at pH 7.5 (E) and after switching back to pH 4.1 (F). Bar=1000 Å.