Further genetic studies of the carQRS region of Myxococcus xanthus

The light-inducible carotenogenic response of Myxococcus xanthus has previously been shown to be controlled by the three genes of the carQRS operon. We describe a function for each of these three genes, and a possible mechanism by which they act in concert to carry out those functions. The promoter of the three genes, previously described as light- inducible, is negatively controlled by the product of the carR gene. We show that this negative control acts through the product of the carQ gene, itself positively required for promoter activity. We present a model for the light-induction of the promoter of the carQRS operon (pQRS). In the dark, the promoter-activating, carQ gene product is sequestered to the membrane by the product of the carR gene. Upon light-induction this sequestration ceases and the CarQ protein is released to cause increased expression of the carQRS operon. There is evidence that, upon light-induction, the CarR protein is actively degraded by a protease, possibly encoded by the carD gene. There is an absolute requirement for the maintenance of translational coupling between the two genes, carQ and carR. The model is extended to show how translational coupling could aid the sequestration of CarQ to the membrane. Only some of the structural genes Ctbose at carB) are activated by the cars gene product. Cars is not required for the activation of pQRS. The structural genes at carC are shown to be activated by the carQ gene product. A preliminary analysis of the structure of pQRS is presented. There appears to be a functional requirement by this promoter for an intact promoter of the upstream and divergently transcribed gene, gufA. An extension to the DNA sequence of the chromosome adjacent to the carQRS operon is presented.