Trans action and the var1 determinant region on yeast mitochondrial DNA. Specific labeling of mitochondrial translation products in zygotes.

We have studied the specification of the apparent molecular weight form of the var1 polypeptide by the var1 determinant region located on mitochondrial DNA of Saccharomyces cerevisiae. A complementation assay has been developed whereby mitochondrial translation products can be labeled selectively in zygotes with 35SO42-. The procedure takes advantage of the fact that met- cys- grande (rho +) strains are unable to incorporate 35SO42- into protein. When such a strain is crossed with a met+ cys petite (rho -) and cells of the mating mixture are labeled with 35SO42- in the presence of cycloheximide under conditions where glycolysis is inoperative, only mitochondrial translation products within zygotes are labeled. Using this procedure with petites containing the var1 determinant region, we find that as much as 75% of the total polypeptide synthesized in zygotes is the form specified by the petite var1 allele. Moreover, mitochondria containing mixed mitochondrial genomes (rho + and rho -) can be isolated from zygotes, and both the rho + and rho - var1 alleles are expressed in vitro. From these results and the analysis of discriminating petites in which var1 recombination can be distinguished from complementation, we conclude that the var1 determinant can specify the apparent molecular weight form of the var1 polypeptide in trans.