A light-inducible promoter (P(B)) drives the carB operon (carotenoid genes) of the bacterium Myxococcus xanthus. A gene encoding a regulator of carotenoid biosynthesis was identified by studying mutant strains carrying a transcriptional fusion to P(B) and deletions in three candidate genes. Our results prove that the identified gene, named carA, codes for a repressor of the P(B) promoter in the dark. They also show that the carA gene product does not participate in the light activation of two other promoters connected with carotenoid synthesis or its regulation in M. xanthus. CarA is a novel protein consisting of a DNA-binding domain of the family of MerR helix-turn-helix transcriptional regulators, directly joined to a cobalamin-binding domain. In support of this, we report here that the presence of vitamin B(12) or some other cobalamin derivatives is absolutely required for activation of the P(B) promoter by light.