Rho¯ Mitochondrial Genomes and Their Influence on Adaptation to Nutrient Stress in Lager Yeast Strains. Liam Good, Terrance M. Dowhanick, José E. Ernandes, Inge Russell, and Graham G. Stewart. J. Am. Soc. Brew. Chem. 51:0035, 1993.

Mitochondrial genomes from two production lager yeast strains were mapped with restriction endonucleases and compared to several spontaneous respiratory-deficient rho¯ mutants arising from these same strains. The maps constructed for the respiratory-sufficient mitochondrial genomes consisted of 68-kilobase pairs, and the arrangements of restriction sites were very similar to each other and to those of other lager strains. The parental maps were used to characterize the aberrant mitochondrial genomes found in several spontaneous respiratory-deficient mutants isolated from the wild-type cultures. Massive deletions were observed in the mitochondrial genomes of the rho¯ isolates, whereas no differences were observed in electrokaryotypes of the mutants to their respective parents. To determine whether these aberrations affected adaptation to nutrient stress, the parents and mutants were pregrown in a nutrient-rich medium and then transferred to minimal medium, maintaining maltose as the sole carbon source. Both parent strains adapted to the nutrient stress and grew exponentially within 24 hr, whereas the respiratory-deficient mutants required two to 18 days. Adaptation times varied among the mutants, except between those that retained the same mutant mitochondrial DNA restriction fragments. Differential sensitivity to nutrient stress in the rho¯ mutants appeared to be influenced by the different mitochondrial DNA mutations.

Keywords: Mitochondrial DNA, Respiratory deficient, Saccharomyces, Stress adaptation, Yeast