Copper induction of carotenoid synthesis in the bacterium Myxococcus xanthus

Copper induces a red pigmentation in cells of the bacterium Myxococcus xanthus when they are incubated in the dark, at suboptimal growth conditions. The colouration results from the accumulation of carotenoids, as demonstrated by chemical analysis, and by the lack of a copper effect on M. xanthus mutants affected in known structural genes for carotenoid synthesis. None of several other metals or oxidative agents can mimic the copper effect on carotenoid synthesis. Until now, blue light was the only environmental agent known to induce carotenogenesis in M. xanthus. As happens for the blue light, copper activates the transcription of the structural genes for carotenoid synthesis through the transcriptional activation of the carQRS operon. This encodes the ECF sigma factor CarQ, directly or indirectly responsible for the activation of the structural genes, and the anti-sigma factor CarR, which physically interacts with CarQ to blocks its action in the absence of external stimuli. All but one of the other regulatory elements known to participate in the induction of carotenoid synthesis by blue light are required for the response to copper. The exception is CarF, a protein required for the light-mediated dismantling of the CarR-CarQ complex. In addition to carotenogenesis, copper induces other unknown cellular mechanisms that confer tolerance to the metal.