Structures of ribosomes from different species. The small subunit is on the left. (a) X-ray structure of the T. thermophilus 70S ribosome . (b) Cryo-EM map of the E. coli 70S ribosome . (c) Cryo-EM map of the yeast 80S ribosome . Expansion regions are darker. The dashed line indicates a flat surface that suggests eukaryotic specialization of 60S subunit for association with a planar membrane. (d) Cryo-EM map of the mammalian mitochondrial ribosome . Reproduced with permission from (a) , (b) , (c)  and (d) .