Negative regulator of genes involved in carotenoid biosynthesis. Requires cobalamin (vitamin B12) for repressor activity. In the dark, binding of cobalamin to CarH induces its oligomerization, which enhances binding to the DNA and repressor activity. Light causes cobalamin photolysis and disruption of the cobalamin-CarH complex, which decreases interaction with DNA and allows transcription of the target genes. Homotetramer in the dark in the presence of cobalamin. Monomer in the light or in the absence of cobalamin. The N-terminal region interacts with DNA, and the C-terminal region is involved in oligomerization and binding of cobalamin. Belongs to the CarA/CarH B12-binding photoregulator family.