Conformation of ribosomes from the vegetative amoebae and spores of Dictyoistelium discoideum.

Ribosomes from two different cell types of D. discoideum--the undifferentiated amoebae and the differentiated spores--display considerable differences in protein composition (Ramagopal and Ennis 1979 and manuscript in preparation). These differences do not affect the three-dimensional structure of monosomes and large (60S) and small (40S) subunits from the two cell types to an extent detectable by sedimentation analysis or electron microscopy. High resolution electron microscopic images of ribosomal particles from the amoebae and the spores are similar and, in general, comparable to that of 80S, 60S, and 40S ribosomal particles from other eukaryotic sources. However, distinct differences in the conformation and stability of the two types of ribosomes are detectable by circular dichroic spectroscopy. The degree of the ordered secondary structure of rRNAs is similar in the 80S monosomes from the amoebae and the spores, but higher in the amoeba subunits. The results of thermal melting experiments show that the small subunit from the spores is more stable than that from the amoebae. The established differences in the conformation of rRNAs, most probably due to the interactions with cell-specific ribosomal proteins, can be responsible for the differences in stability of ribosomes from the two cell types of slime mold.