Cryo-electron microscopy of helical particles TMV and T4 polyheads.

Vitrified Tobacco Mosaic Virus (TMV) and T4 polyhead suspensions have been studied by cryo-electron microscopy. The order of TMV particles is preserved to distances better than 0.3 nm when vitrified. In spite of the high beam sensitivity of unstained biological material, the resolution of images extends to 1.15 nm. Radial density distributions calculated from images of TMV and the helical aggregate of TMV protein (TMVP) show that the RNA is likely visualized in the virus. However, due to the imaging conditions of unstained vitrified specimens, the position of the RNA is erroneous. While TMV and TMVP do not show any departure from cylindrical symmetry, T4 polyheads are sensitive to the thickness of the ice layer in which they are embedded. To avoid flattening, T4 polyheads have to be embedded in an ice layer thicker than their diameter.