Figure 2. The translocation of mtDNA fragments to the nucleus affects the yeast chronological life span.

The strains were grown in defined, synthetic glucose-containing medium. To determine the total numbers of viable cells during the CLS, appropriate dilutions of cells were spread on plates containing rich medium (YEPD) at the indicated days (panels A, D, G, and H). To determine the mtDNA transfer frequency, 5x10⁷ cells were plated on plates lacking tryptophan on the same days (panels B and E). The numbers of colonies obtained from those plates were then normalized to the numbers of viable cells. The respiration efficiency is expressed as a ratio of the number of colonies obtained from glycerol-containing plates (respiring cells) to the number of colonies obtained from glucose-containing plates (total viable cells) (panels C and F). The strains shown in panels A to F contain functional mtDNA (rho⁺), whereas the strains shown in panels G and H lack mtDNA (rho⁰). Standard errors of three independent experiments are shown. The significance in the difference in the CLS was determined by a Student’s t-test (P < 0.005 (*) for day 6 in panel A, P < 0.01 (**) for day 13 in panel D, P < 0.4 (***) for day 2 in panel G, and P < 0.01 (****) for day 4 in panel H).