Three-dimensional reconstruction of the 30 S ribosomal subunit from randomly oriented particles

Electron micrographs show the small (30 S) subunit of Escherichia coli ribosomes lying in a wide range of positions on the specimen support, related by rotation principally around the long axis of the particle. Through correspondence analysis, a multivariate statistical method that distinguishes the major factors accounting for interimage variance, the (aligned) views of the randomly oriented particles were ordered and grouped according to tilt angle. Views so grouped were then averaged and used as input to a three-dimensional reconstruction program. The particle reconstructed from nine averaged projections spanning a 160 degrees rotational range has a resolution of 5 nm in planes perpendicular to the long axis of the particle and approximately 3 nm in the direction of the long axis. It is somewhat asymmetrical and quite compact; its most conspicuous feature is the "platform" that wraps partially around the middle of the subunit.