Electron microscopic identification of the yeast spliceosome.

We have partially purified the yeast spliceosome by differential sedimentation in glycerol gradients. By electron microscopy we have identified a particle in these fractions that is the spliceosome. In 100 mM KCl buffer, the yeast spliceosome is an ovoid disc with the dimensions of 20 x 23.5 nm with a central indentation. To verify that these ovoid particles were spliceosomes, specific labels were used to tag them. These tagged spliceosomes were then identified in the electron microscope. The salt dependent shift of sedimentation rate for the spliceosome can be explained by a change in size of the particle.