Physical mapping of genetic determinants on yeast mitochondrial DNA affecting the apparent size of the Var 1...

The var1 locus on yeast mitochondria) DNA is defined by a set of alleles that affect the apparent size (Mr 40,000-44,000) of a mitochondria) translation product (var1), a protein of the mitochondria) 38S ribosomal subunit. Using petite deletion and restriction endonuclease site mapping, we have defined the physical location of all var1 alleles to a specific restriction fragment of roughly 2.1 kbp located between the antibiotic resistance loci ery1 and olil. Surprisingly, from base sequence studies of this region by Tzagoloff et al. (1980), it appears that the DNA fragment we have mapped contains little or none of the structural gene for the var1 protein since the fragment is composed primarily of long stretches of dA + dT interspersed with short clusters high in dG + dC. Nevertheless, by a type of complementation test termed zygotic gene rescue (Strausberg and Butow, 1977), we show that with petites retaining that restriction fragment and short flanking sequences, var1 polypeptide characteristic of the strain from which the petite was derived, is expressed in zygotes formed between the petite and a wild-type tester. Thus the ability of the var1 determinant to act in trans suggests that control of expression of different var1 species involves intermolecular interactions, perhaps at the level of RNA splicing. Our results are discussed in terms of several possible models for the organization and control of the var1 structural gene.