Mitochondria1 Asparaginyl‐tRNA Synthetase Encoded by the Yeast Nuclear Gene YCR24c

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Abstract
One of the open reading frames located on yeast Saccharomyces cerevisiae chromosome III, YCR24c, appeared to code for a protein of unknown function, but the predicted sequence showed similarity with asparaginyl-tRNA synthetase from Escherichiu coli, with 3896 amino acid identity. There is a putative mitochondrial targeting signal at the N-terminus of the YCR24c product. Northern blot analysis of total RNA from a wild-type strain Σ1278b confirmed that YCR24c was transcribed. Disruption of the chromosomal copy of YCR24c in a respiratory-competent haploid cell induced a petite phenotype, but did not affect cell viability. This respiratory-defective phenotype is typical for a mutation in a nuclear gene that induces a non-functional mitochondrial protein synthesis system. The protein encoded by YCR24c was expressed in Escherichia coli in a histidine-tagged form and isolated. The enzyme aminoacylated unfractionated Escherichiu coli tRNA with asparagine. These results identified YCR24c as the structural gene for yeast mitochondrial asparaginyl-tRNA synthetase.