ASSEMBLY OF THE MITOCHONDRIAL-MEMBRANE SYSTEM - 2 SEPARATE GENES CODING FOR THREONYL-TRANSFER RNA IN THE MITOCHONDRIAL-DNA OF SACCHAROMYCES-CEREVISIAE

Mitochondria of S. cerevisiae contain 2 tRNA''s that are acylated with threonine. The 2 isoaccepting species .**GRAPHIC**. and .**GRAPHIC**. can be separated by reversed-phase chromatography on RPC-5. A cytoplasmic mutant was isolated which lacks .**GRAPHIC**. but has normal levels of .**GRAPHIC**. This mutation was previously shown to map between the oxi 1 and oxi 2 loci on mitochondrial DNA. .**GRAPHIC**. and .**GRAPHIC**. hybridize to wild type mitochondrial but not nuclear DNA and can partially compete with each other. Hybridization of each species to different segments of mitochondrial DNA isolated from .rho.- clones indicate that there are 2 threonyl tRNA genes. One gene is located between oxi 1 and oxi 2 and codes for .**GRAPHIC**. The 2nd gene codes for .**GRAPHIC**. and is near the cap locus. Binding assays to Escherichia coli ribosomes indicate that .**GRAPHIC**. recognizes the threonine triplet ACA and may also recognize the other 3 triplets but with a much lower efficinecy. None of the 4 codons for threonine stimulate the binding of .**GRAPHIC**. to the ribosomes.