Ribosomes of Trypanosoma brucei, a parasitic, flagellated protozoan (order Kinetoplastida), were identified on sucrose density gradients by their radioactively labeled nascent peptides. Ultraviolet absorption revealed only cytoplasmic ribosomes which served as internal sedimentation markers. Synthesis on cytoplasmic ribosomes was completely inhibited by cycloheximide. In the presence of this antibiotic, nascent peptides were associated with ribosomes of lower sedimentation coefficient than the cytoplasmic ribosomes. Chloramphenicol blocked synthesis on these ribosomes which are probably the mitochondrial ribosomes. These ribosomes differed from the cytoplasmic ribosomes in several ways. Their sedimentation coefficient was about 72S rather than 84S. The stability of the 72S ribosomes was less sensitive to pancreatic ribonuclease and low Mg-++ concentrations, dissociating below 0.1 mM Mg++. The 72S ribosomes were more sensitive to elevated KCl concentrations, dissociation above 0.25 M. Protein synthetic activity associated with the 72S class of ribosomes was found in trypanosomes grown in rats. Under these conditions no cytochromes or fully active Krebs cycle is present in these cells and respiration is insensitive to cyanide.

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