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. Binds cobalamin (vitamin B12), but cobalamin is not required for CarA activity. Interaction with CarS prevents binding to DNA. Forms homodimers or 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. Deletion causes light-independent expression from the carB promoter. Belongs to the CarA/CarH B12-binding photoregulator family.