Amphimeric mitochondrial genomes of petite mutants of yeast. I. Flip-flop amphimers make up the mitochondrial genomes of ``palindromic'' petite mutants of yeast

Abstract

 The mitochondrial (mt) genomes of three spontaneous cytoplasmic ``palindromic'' petite mutants of yeast were studied by restriction-enzyme analysis. These mt genomes were shown to be made up of an amplified ``master basic unit'' consisting of two inverted segments (a and A) and of two different unique segments (d and t) separating them. The basic unit was called ``amphimeric'', this term having been first proposed for certain lambda-phage mutants. We propose that in the mt genomes of the petite mutants studied, the four possible variants of the amphimeric basic unit form two – ``flip'' and ``flop'' – tetra-amphimeric repeat units datA-datA-DaTA-DaTA and DatA-DatA-daTA-daTA, respectively. These repeat units make two types of ``amphimeric'' mt genomes which exist in equal proportions in the cell. In each mt genome, the duplicated segment regularly alternates in its direct and inverted orientation (a…A…a…A…), whereas the unique segments are arranged twice in tandem fashion and twice in inverted fashion (d…d…D…D…d…d… and t…t…T…T…t…t….). The only difference between flip and flop amphimeric mt petite genomes is the different relative orientation of the unique segments in the mono-amphimers. In the mono-amphimers of flip mt genomes, both unique segments are arranged in the same direction (d…t and D…T), whereas in the mono-amphimers of flop mt genomes, both unique segments are arranged in opposite directions (D…t and d…T). Control experiments on one spontaneous petite mutant (which was an ancestor of the mutants studied here) and on three independent, previously investigated, EtBr-induced mutants showed that all of them were, in fact, organized in the same way. Analysing our experimental data and the results published by others, we conclude that amphimeric organization is a general feature of mt petite genomes of yeast previously called ``palindromic'' or ``rearranged''.