Catalyzes the synthesis of alpha-ribazole-5'-phosphate from nicotinate mononucleotide (NAMN) and 5,6-dimethylbenzimidazole (DMB). 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. Belongs to the CobT family.