Loss of cytochrome c oxidase activity and acquisition of resistance to quinone analogs in a laccase-positive variant of Azospirillum lipoferum

Laccase, a p-diphenol oxidase typical of plants and fungi, has been found r ecently in a proteobacterium, Azospirillum lipoferum. Laccase activity was detected in both a natural isolate and an in vitro-obtained phase variant t hat originated from the laccase-negative wild type. In this study, the elec tron transport systems of the laccase-positive variant and its parental lac case-negative forms were compared. During exponential (but not stationary) growth under fully aerobic (but not under microaerobic) conditions, the lac case-positive variant lost a respiratory branch that is terminated in a cyt ochrome c oxidase of the aa(3) type; this was most likely due to a defect i n the biosynthesis of a heme component essential for the oxidase. The lacca se-positive variant was significantly less sensitive to the inhibitory acti on of quinone analogs and fully resistant to inhibitors of the be complex, apparently due to the rearrangements of its respiratory system. We propose that the loss of the cytochrome c oxidase-containing branch in the variant is an adaptive strategy to the presence of intracellular oxidized quinones, the products of laccase activity.