NUCLEAR MUTATIONS AND MITOTIC RECOMBINATION IN SACCHAROMYCES BY LIGHT-ACTIVATED ETHIDIUM AZIDES

Ethidium mono- and diazide analogs have been used as photoaffinity probes to study the mechanism of the ethidium-induced petite mutation in yeast [7,10,14]. The azide moiety when exposed to light is converted to a reactive nitrine intermediate. The diradical nitrene effects a covalent attachment to sites of interaction bound reversibly by the drug. Ethidium azide photoaffinity labeling has been used to verify the prerequisite covalent attachment of ethidium to mitochondrial DNA to bring about the petite mutation in yeast [6]. Bastos has also reported a specific photoattachment of ethidium azide to a polypeptide (subunit 9) of the membrane bound ATPase in yeast mitochondria [1]. Isolated DNA from yeast cells treated in vivo with [14C]ethidium monoazide plus light contained covalent adducts on both nuclear and mitochondrial DNA, although the specific radioactivity of mitochondrial DNA was more than 10 times higher than nuclear DNA [11]. Drug distribution studies of [14C]ethidium monoazide have indicated that greater than 50% of the covalent adducts in the nuclear centrifugation fraction (2000 × g) reside on nuclear proteins [3]. Nuclear damage in yeast by the photolysis of ethidium azides is apparent from the increased killing effect in cells treated with ethidium azide derivatives plus light as opposed to dark-treated cells [7,10,11]. This communication reports a concomitant increase in nuclear mutations and genetic activity from the photoactivated covalent attachment of ethidium azides to nuclear components. © 1979.