A CLUSTER OF STRUCTURAL AND REGULATORY GENES FOR LIGHT-INDUCED CAROTENOGENESIS IN MYXOCOCCUS-XANTHUS

In the bacterium Myxococcus xanthus, several genes for carotenoid synthesis lie together at the carA-carB chromosomal locus and are co-ordinately activated by blue light. A 12-kb DNA stretch from wildtype M. xanthus has been sequenced that includes the entire carA-carB gene cluster. According to sequence analysis, the cluster contains 11 different genes. Intergenic distances are very short or nil (implying translational coupling), giving further support to previous evidence indicating that most (or all) of the genes in the cluster form a single operon. At the promoter region, a potential -35 site for the binding of sigma factors is found. However, the -10 region shows little similarity with analogous sites in other bacterial promoters. Five (possibly six) genes in the carA-carB operon code for enzymes acting on early or late steps of the pathway for carotenoid synthesis. Other genes in the operon show no overall similarity with previously known genes. However, peptide stretches in the predicted products of two genes exhibit strong similarity with the DNA binding domain of the MerR family of transcriptional regulators. At least one of the predicted DNA-binding domains is altered in a mutant strain affected in light-regulation of the car genes.