Catalyzes the isomerization of 5-deoxy-alpha-D-ribose 1-phosphate to 5-deoxy-D-ribulose 1-phosphate, as part of a 5-deoxyribose salvage pathway that recycles this toxic radical SAM enzyme by-product to mainstream metabolites. 5-deoxy-alpha-D-ribose 1-phosphate = 5-deoxy-D-ribulose 1-phosphate Carbohydrate degradation. Belongs to the EIF-2B alpha/beta/delta subunits family. DrdI subfamily.