The structure of ribosomes as indicated by studies of tetramers from hypothermic chick embryos.

Ribosome tetramers induced in chick embryos by exposure to cold, and tetramers of large subunits derived from them, have been studied by electron microscopy and sucrose-density-gradient analysis. Individual ribosomes of the normal tetramer are elongated bean-shaped structures, 220-280A by 195A (1A=10(-1)nm) with a cleft in the outer edge which divides the two-dimensional image into two unequal ends. Most of the tetramers appear to attach to the surface of the electron-microscope grid by one preferred face. The subunits of the large-subunit tetramers have a round outline and no cleft. About 25% of the subunits of these tetramers have a line running radially across the particle. The dissociation of tetramers into large-subunit tetramers and small subunits has been shown to be reversible. Mixtures of these particles from sucrose-density-gradient fractions were reassociated to give a tetramer with the same sedimentation coefficient as the original tetramer and with the same structure as viewed in the electron microscope. The results indicate that the cleft is a property of the complete ribosome, and that it marks the position of the small subunit. The reversibility of the dissociation also strengthens the view that no change in the large subunit occurs during dissociation or reassociation, i.e. that the sites of interaction between ribosomes in both types of tetramer are the same. The conclusions affect the interpretation of electron-micrograph images and an anomaly in the relationship between the two types of tetramer is discussed.