Assembly of the mitochondrial membrane system

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Abstract
Summary 1. Mitochondria of Saccharomyces cerevisiae contain two tRNA's that are acylated with threonine. The two isoaccepting species (tRNA Thr1 and tRNA Thr2 ) can be separated by reversed-phase chromatography on RPC-5. 2. A cytoplasmic mutant has been isolated which lacks tRNA Thr1 but has normal levels of tRNA Thr2 . This mutation was previously shown to map between the oxi 1 and oxi 2 loci on mitochondrial DNA. 3. tRNA Thr1 and tRNA Thr2 hybridize to wild type mitochondrial but not nuclear DNA and are capable of partially competing with each other. Hybridization of each species to different segments of mitochondrial DNA isolated from ρ¯ clones indicate that there are two threonyl tRNA genes. One gene is located between oxi 1 and oxi 2 and codes for tRNA Thr1 . The second gene codes for tRNA Thr2 and is near the cap locus. 4. Binding assays to E. coli ribosomes indicate that tRNA Thr2 recognizes the threonine triplet ACA and may also recognize the other three triplets but with a much lower efficiency. None of the four codons for threonine stimulate the binding of tRNA Thr1 to the ribosomes.

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