respiratory metabolism petite-negative APO:0000154 The petite-negative phenotype arises from mutation in a nuclear gene that is not essential in an otherwise wild-type strain with an intact mitochondrial genome, but becomes essential in strains lacking the mitochondrial genome, referred to as rho- or rho0. Petite-negative strains cannot grow on fermentable carbon sources (e.g., glucose) in the absence of an intact mitochondrial genome. This phenotype is often assayed as the inability grow in the presence of ethidium bromide, which causes the loss of the mitochondrial genome. YPO:0000154 observable