A nuclear mutation decreasing rho- production modifies the unstable nitrous-acid sensitivity of yeast.

The nuclear mmgl mutation, which reduces rho- mutability in Saccharomyces cerevisiae, renders the rho+ cells less sensitive to inactivation by nitrous acid (NA) but has little or no effect on the NA sensitivity of the rho0 cells devoid of mitochondrial (mt) DNA. Therefore the cells' NA sensitivity seems to be influenced by an interaction of the mmgl mutation and the mt genome rather than the mmgl mutation itself. The clonal variation of NA sensitivity is high in MMG+ yeast and significantly reduced in rho0 mutants and mmgl cells. The results presented suggest that frequent spontaneous heritable changes of the mt genome occur in MMG+ cells, which, (i) unlike rho- mutations, do not damage the respiratory capacity, and (ii) manifest themselves in a high clonal variation of NA sensitivity.