Mammalian mitochondria contain 55S ribosomes of unusual composition and physical-chemical properties. The objective of this research project is the elucidation of the structure, composition, function and biosynthesis of mammalian mitochondrial ribosomes to allow a comprehensive comparison of 55S ribosomes with extramitochondrial ribosomes. Mitochondrial and microsomal ribosomes prepared on a large scale from bovine liver will be used, with E. coli 70S ribosomes, in comparative studies of antibiotic sensitivity and ribosome function in cell-free systems. The ribosomal proteins of 55S and extramitochondrial ribosomes will be enumerated and characterized by polyacrylamide gel electrophoresis in two dimensions. These analyses will include studies of MW, stoichiometry, amino acids composition, topographical distribution and functional assignments. Experiments will also be performed to ascertain which, if any of the mitochondrial r-proteins are made in mitochondria, and also to determine functional and immunochemical homologies between r-proteins of 55S and 70S E. coli ribosomes.