ELECTRON CRYOMICROSCOPY OF BACILLUS-STEAROTHERMOPHILUS 50-S RIBOSOMAL-SUBUNITS CRYSTALLIZED ON PHOSPHOLIPID MONOLAYERS

50 S ribosomal subunits from Bacillus stearothermophilus have been crystallized as 2-dimensional periodic arrays on phospholipid monolayer films at the water-air interface. These crystals were presented in vitreous ice and imaged with 100 keV electrons under low dose and low temperature conditions. The unit cell parameters of the crystals are a = 371.3(± 3.8)Å, b = 152.3(± 1.6)Å, Γ = 96.3(± 1.0)°. Some of the image arrays of these crystals have twofold rotational symmetry with a phase residual of less than 25°. The mean figure of merit of the merged structure factors from these image arrays out to 20 Å resolution is higher than 0.87. The 2-dimensional projection map shows a level of detail not seen in previous structural studies of the 50 S ribosome subunit. Some of these features may be related to the current 3-dimensional model of the subunit. This analysis illustrates the potential of using the electron crystallographic approach for determining the 3-dimensional structure of the 50 S ribosomal subunit crystallized on a monolayer surface. In addition, the structural information retrieved by electron crystallography might be useful for phasing X-ray data towards an atomic resolution model of the ribosome. © 1994 Academic Press Limited.