Negative regulator of the carB operon in the dark. Binds specifically to the CarA operator, in the region around the carB promoter, which blocks access to the RNA polymerase. 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 carB operon. Interaction with CarS also prevents binding to DNA. Forms oligomers. Interacts with CarS. The N-terminal region interacts with DNA and CarS, and the C-terminal region is involved in oligomerization and binding of cobalamin. Belongs to the CarA/CarH B12-binding photoregulator family.