Genic and allelic interactions in the carotenogenic response of myxococcus xanthus to blue light.

In the bacterium Myxococcus xanthus, the synthesis of carotenoids requires illumination with blue light. This stimulates transcription of the carB locus, which is positively required for carotenogenesis. Mutations at the carR locus and the only mutation known at carA cause constitutive expression of carB and thus remove the light requirement for carotenoid accumulation (Car(c) phenotype). The carR locus is unlinked, and carA is linked, to carB. We have now identified a novel class of car mutation, closely linked to carR, that block accumulation of carotenoids and transcription of carB, and are epistatic over the Car(c) mutations at carR but not over the Car(c) mutation at carA. We also report here the cloning of a 16-kb DNA fragment that contains the entire carA gene in a shuttle vector for DNA transfer between Escherichia coli and M. xanthus. The study of allelic interactions at this gene strongly indicate that carA is a cis-acting element for the control of carB expression. A regulatory model that satisfies all the indicated data is presented.