Mitochondrial membranes and mutagenesis by ethidium bromide

The conversion of wild type (ρ⁺) to cytoplasmic petites (ρ^?) in Saccharomyces cerevisiae, à mutation in mitochondrial DNA, can be brought about with high efficiency by low concentrations of ethidium bromide (EB). The rate and extent of mutagenesis and its expression can be influenced, and even reversed, by a number of genetic lesions, agents or treatments affecting mitochondrial structure and metabolism. Among them are incubation at 45°, exposure to Antimycin A, growth on different carbon sources and the presence or absence of 2 different gene products previously implicated in the repair of UV induced lesions in mitochondrial DNA. Based on these observations a model for EB mutagenesis is advanced which postulates a complex between mitochondrial DNA and the inner membrane as the target susceptible to modification by EB. This model predicts that altered membranes should lead to changes in the susceptibility of cells to the mutagenic action of EB. This prediction has been verified by comparing cells that contain one of 2 structurally quite distinct monounsaturated C₁₈ fatty acids in their mitochondrial phospholipids: greater resistance to mutagenesis and ease of thermal protection is exhibited when cells – and mitochondria – contain oleic (Δ⁹cis, m.p. < 5°) rather than petroselinic (Δ⁶cis, m.p. 28°) acid in their phospholipids. As a corollary, studies on EB mutagenesis and mitochondrial DNA may be used as probes for the mitochondrial inner membrane to reveal some perhaps novel functions.     

The role of development and adrenal steroids in the regulation of the mineralocorticoid receptor messenger RNA.

The ontogeny, adrenal-feedback regulation and regional distribution of the mineralocorticoid receptor (MR) mRNA were examined in the rat brain and kidney. In the kidney, MR mRNA levels in the adult were only 25-30% of the neonatal concentration. Adrenalectomy caused a 35% increase in total brain MR mRNA and a 94% increase in kidney MR mRNA levels. Examination of the regional distribution of the MR mRNA within the brain revealed that the hippocampus had the highest levels, and the mRNA abundance increased after adrenalectomy. The administration of dexamethasone to intact animals resulted in a significant reduction of MR mRNA in the kidney of neonatal rats but not in the brain. These data indicate that there are developmental changes in MR gene expression in kidney and that adrenal steroids can modulate MR gene expression in both the brain and kidney.