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Light-induced carotenogenesis in Myxococcus xanthus is under the control of the carQRS operon. CarQ, a proposed extracytoplasmic (ECF) RNA polymerase sigma factor, is required for expression of the operon and the carC gene that encodes phytoene dehydrogenase. CarR, an inner membrane protein in Escherichia coli, is essential for carQRS promoter inactivation in the dark. CarS is required for the light-dependent expression of the promoter of the carB gene cluster that encodes the rest of the structural genes for carotenogenesis. Regulation of carQRS is dependent on the stoichiometry of CarQ and CarR. Increasing the copy number of carQ over carR led to constitutive carotenogenesis, as did loss of translational coupling between carQ and carR. The severity of the constitutive phenotype depended on the distance between the uncoupled genes. When expressed in M. xanthus, a CarR:beta-galactosidase fusion protein 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.

Original publication




Journal article


Mol Microbiol

Publication Date





171 - 186


Bacterial Proteins, Blotting, Southern, Carotenoids, Electrophoresis, Polyacrylamide Gel, Escherichia coli, Gene Dosage, Gene Expression Regulation, Bacterial, Lac Operon, Light, Membrane Proteins, Models, Molecular, Myxococcus xanthus, Phenotype, Promoter Regions, Genetic, Protein Biosynthesis, Recombinant Fusion Proteins, Sigma Factor, beta-Galactosidase