Several different but interrelated lines of research into the structure-function relationships of ribosomes and transfer RNA are being pursued. One phase deals with the characterization of mitochondrial ribosomes (mt ribosomes) from cultured mammalian cell lines. Properties of the ribosomal proteins (r-proteins) and the ribosomal RNA (rRNA) of mt ribosomes from HeLa cells resistant to chloramphenicol (CAP), as the result of a cytoplasmically inherited mutation, will be compared to their CAP-sensitive counterparts, to identify the gene product altered by mutation. These results will help develop a better understanding of the relative contributions of the nuclear and mitochondrial genomes to mt ribosome biogenesis. Additional studies involve the use of affinity binding techniques developed earlier to identify and isolate the eukaryotic r-proteins (yeast, HeLa, etc.) which bind specifically to 5s and 5.8s RNA covalently coupled to agarose. Comparison with E. coli r-proteins binding to bacterial 5s RNA should permit recognition of functionally analogous eukaryotic and prokaryotic r-proteins and help clarify the relationships among bacterial 5s RNA and eukaryotic 5s and 5.8s RNA. The complexes between eukaryotic r-proteins and 5s and 5.8s RNA will be further characterized. An additional area of investigation involves 19F NMR studies with E. coli transfer RNAs having most of their uridine-derived minor nucleosides replaced by 5-fluorouridine. The spectra of these tRNAs and complexes of the tRNAs with various proteins, e.g., aminoacyl tRNA synthetases, EFTu or initiation factors, will provide evidence relating to the solution structure of tRNA and possible changes in this structure on interaction with proteins. Analysis of 19F NMR spectra should permit identification of specific protein recognition sites on tRNA. The functional roles of modified base constituents in tRNA will also be investigated by studying the activity of purified 5-fluorouracil-substitutes tRNA in the initiation of protein synthesis and in codon recognition.