Investigation of the 50 S ribosomal subunit by electron microscopy and image analysis.

In electron micrographs of 50 S (large) subunits from Escherichia coli ribosomes, the highly preferred crown view is inferred to represent the roughly hemispherical particle lying with its flat or concave face against the carbon film. Single particle averaging allows the reproducible details of the crown view particle to be recognized. Multivariate image analysis shows the most variable morphological features of this view to be the two side protrusions, the L7/L12 stalk and the L1 ridge, both of which show apparent positional variations. The invariance of the features of the particle body implies that the movements of the side protrusions are not merely a result of perspective changes produced by major rotations of the particle body out of its quasistable, flat-lying position. A bending point localized on the L7/L12 stalk is conjectured to represent a functional "hinge" that may be related to the secondary/tertiary structure of the L7/L12 dimeric protein.