SIGNALS REGULATING THE EXPRESSION OF THE NUCLEAR GENE ENCODING ALTERNATIVE OXIDASE OF PLANT-MITOCHONDRIA

Suspension cells of tobacco (Nicotiana tabacum L. cv Bright Yellow) we re used to investigate signals regulating the expression of the nuclea r gene Aox1 encoding the mitochondrial alternative oxidase (AOX) prote in responsible for cyanide-resistant respiration in plants. We found t hat an increase in the tricarboxylic acid cycle intermediate citrate ( either after its exogenous supply to cells or after inhibition of acon itase by monofluoroacetate) caused a rapid and dramatic increase in th e steady-state level of Aox1 mRNA and AOX protein. This led to a large increase in the capacity for AOX respiration, defined as the amount o f salicylhydroxamic acid-sensitive O-2 uptake by cells in the presence of potassium cyanide. The results indicate that citrate may be an imp ortant signal metabolite regulating Aox1 gene expression. A number of other treatments were also identified that rapidly induced the level o f Aox1 mRNA and AOX capacity. These included short-term incubation of cells with 10 mM acetate, 2 mu M antimycin A, 5 mM H2O2, or 1 mM cyste ine. For some of these treatments, induction of AOX occurred without a n increase in cellular citrate level, indicating that other signals (p ossibly related to oxidative stress conditions) are also important in regulating Aox1 gene expression. The signals influencing Aox1 gene exp ression are discussed with regard to the potential function(s) of AOX to modulate tricarboxylic acid cycle metabolism and/or to prevent the generation of active oxygen species by the mitochondrial electron tran sport chain.