Abstract
In yeast mitochondria, most of the isoaccepting species of tyrosyl tRNA are coded by a mitochondrial gene, tyrA. A particular isoaccepting species is coded by a second mitochondrial gene, tyrB. This gene is not expressed in certain strains of yeast which show no deficient phenotype. Genetic crosses between strains expressing or not expressing the tyrB gene demonstrate that expression is controlled by specific nuclear genes and that a mutation of the tyrA gene can be bypassed when the tyrB gene is operative.