Light induction of gene expression in Myxococcus xanthus

The synthesis of carotenoids by Myxococcus xanthus requires illumination with blue light. Mutations at two loci (carA and carR) remove the blue-light requirement and cause constitutive production of carotenoids. Mutations at a different locus (carB) prevent carotenogenesis in both wild-type and constitutive mutant strains. We describe here three independent car mutations induced by insertion of Tn5 lac, a transposon that carries a transcriptional probe for exogenous promoters. All three transposon insertions block carotenogenesis even in constitutive mutant strains. One insertion is in a previously unknown car gene and the other two are in the carB locus. One of the carB insertions expresses beta-galactosidase at very low levels in the dark but is strongly activated by light. When this Tn5 lac insertion is introduced in carA or carR mutants it expresses beta-galactosidase in dark- as well as light-grown cells. We conclude that carotenogenesis in M. xanthus is activated at the level of transcription by a light-induced mechanism in which the carA and the carR loci (or their gene products) take part. The potential usefulness of M. xanthus as a simple and sensitive tool for studies in photobiology is discussed.