To induce random mitochondrial mutagenesis yeast, resulting in respiration deficiency, ethidium bromide (EtBr) was used. As show in fig.1, and overnight culture of S.cerevisiae 7982 was transferred to fresh medium, and the initial cell density was adjusted to OD600 = 0.2 when the cell density reached at 1.0 of OD600, 0.5 ug/ml EtBr was added to the culture. then, 100 ul of inoculum was spread onto plates and incubated at 42 C, After an initial screening of thermotolerant mutant mutants according to larger colony size at 42 C, a spot assay was used to screen thermotolerant mutants. Briefly, 4-ul aliquots of mutants and control cells grown to OD600 = 1.0 were 10-fold serially diluted, and spotted onto the plates. the plates were then incubated at 30 C and 42 C for 1-2 days.