LIGHT-INDUCED CAROTENOGENESIS IN MYXOCOCCUS-XANTHUS - LIGHT-DEPENDENTMEMBRANE SEQUESTRATION OF ECF SIGMA-FACTOR CARQ BY ANTI-SIGMA FACTOR CARR

Light-induced carotenogenesis in Myxococcus xanthus is under the contr ol of the carQRS operon. CarQ, a proposed extracytoplasmic (ECF) RNA p olymerase sigma factor, is required for expression of the operon and t he carC gene that encodes phytoene dehydrogenase, CarR, an inner membr ane protein in Escherichia coli, is essential for carQRS promoter inac tivation in the dark. CarS is required for the light-dependent express ion of the promoter of the carB gene cluster that encodes the rest of the structural genes for carotenogenesis. Regulation of carQRS is depe ndent on the stoichiometry of CarQ and CarR. Increasing the copy numbe r of carQ over carR led to constitutive carotenogenesis, as did loss o f translational coupling between carQ and carR. The severity of the co nstitutive phenotype depended on the distance between the uncoupled ge nes. When expressed in M. xanthus, a CarR:beta-galactosidase fusion pr otein disappeared in the light. We propose that anti-sigma factor CarR sequesters CarQ to the membrane in the dark, but, in the light, loss of CarR leads to release of the sigma factor.