CORRELATION OF THE EXPANSION SEGMENTS IN MAMMALIAN RIBOSOMAL-RNA WITHTHE FINE-STRUCTURE OF THE 80-S RIBOSOME - A CRYOELECTRON MICROSCOPIC RECONSTRUCTION OF THE RABBIT RETICULOCYTE RIBOSOME AT 21 ANGSTROM RESOLUTION

Samples of 80 S ribosomes from rabbit reticulocytes were subjected to electron cryomicroscopy combined with angular reconstitution. A three- dimensional reconstruction at 21 Angstrom resolution was obtained, whi ch was compared with the corresponding (previously published) reconstr uction of Escherichia coli 70 S ribosomes carrying tRNAs at the A and P sites. Ln the region of the intersubunit cavity, the principal featu res observed in the 70 S ribosome (such as the L1 protuberance, the ce ntral protuberance and A site finger in the large subunit) could all b e clearly identified in the 80 S particle. On the other hand, signific ant additional features were observed in the 80 S ribosomes on the sol vent sides and lower regions of both subunits. Ln the case of the smal l (40 S) subunit, the most prominent additions are two extensions at t he base of the particle. By comparing the secondary structure of the r abbit 18 S rRNA with our model for the three-dimensional arrangement o f E. coli 16 S rRNA, these two extensions could be correlated with the rabbit expansion segments (each totalling ca 170 bases) in the region s of helix 21, and of helices 8, 9 and 44, respectively. A similar com parison of the secondary structures of mammalian 28 S rRNA and E. coli 23 S rRNA, combined with preliminary modelling studies on the 23 S rR NA within the 50 S subunit, enabled the additional features in the 60 S subunit to be sub-divided into five groups. The first (corresponding to a total of ca 335 extra bases in helices 45, 98 and 101) is locate d on the solvent side of the 60 S subunit, close to the L7/L12 area. T he second (820 bases in helices 25 and 38) is centrally placed on the solvent side of the subunit, whereas the third group (totaling 225 bas es in helices 18/19, 27/29,52 and 54) lies towards the L1 side of the subunit. The fourth feature (80 bases in helices 78 and 79) Lies withi n or close to the L1 protuberance itself, and the fifth (560 bases in helix 63) is located underneath the L1 protuberance on the interface s ide of the 60 S subunit. (C) 1998 Academic Press Limited.