Chloramphenicol-induction of a second cyanide-and azide-insensitive mitochondrial pathway in Ustilago maydis

Growth of wild type Ustilago maydis in presence of chloramphenicol results in increased resistance of cyanide-insensitive respiration to hydroxamic acid derivatives. The biphasic nature of the Dixon plots for hydroxamate inhibition of mitochondrial alternative respiration suggested involvement of two cyanide-sensitive systems. This hypothesis was confirmed by the analysis of kinetic data. One of the two systems exhibits normal sensitivity to hydroxamates and appears similar to that present in mitochondria from cells grown in control medium. In contrast, the inducible system is characterised by resistance to hydroxamates and lower substrate affinity. With excess substrate and AMP in reaction mixutre the two systems have equal participation in the cyanide-insensitive respiration of chloramphenicol-treated mitochondria. In the absence of AMP, however, the inducible oxidase seems to exhibit much higher affinity for oxygen. The new system appears responsible also for changes in sensitivity of alternative respiration to low temperatures. It differs from the hydroxamate resistant systems recently reported for other fungi in its high activity and its complete insensitivity to azide and cyanide.