Catalyzes the synthesis of alpha-ribazole-5'-phosphate from nicotinate mononucleotide (NAMN) and 5,6-dimethylbenzimidazole (DMB) (PubMed:8206834, PubMed:10587435, PubMed:11441022, Ref.7, PubMed:24121107). Able to use a variety of other nucleotide bases as substrate to create alternative lower ligands for cobamide (PubMed:11441022, Ref.7, PubMed:24121107). 5,6-dimethylbenzimidazole + nicotinate beta-D-ribonucleotide = alpha-ribazole 5'-phosphate + H(+) + nicotinate Nucleoside biosynthesis; alpha-ribazole biosynthesis; alpha-ribazole from 5,6-dimethylbenzimidazole: step 1/2. Homodimer. No growth on cobinamide (CBI); double cobB-cobT deletion mutants do not grow on CBI and DMB (PubMed:8206834). In vitro assays of this enzyme are performed at pH 10 because at physiological pH (7.0) its activity is undetectable; this makes it difficult to assess the role of mutagenesis of potential proton acceptor active site residues (Glu-174 and Glu-317) (PubMed:24121107). Belongs to the CobT family. Extended N-terminus. Extended N-terminus.