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. Also seems to be able to catalyze the conversion of methylthioribose-1-phosphate (MTR-1-P) into methylthioribulose-1-phosphate (MTRu-1-P). However this enzyme may not function in methionine salvage in B.thuringiensis since it exists a paralog (MtnA) present in the methionine salvage pathway cluster. 5-deoxy-alpha-D-ribose 1-phosphate = 5-deoxy-D-ribulose 1-phosphate S-methyl-5-thio-alpha-D-ribose 1-phosphate = S-methyl-5-thio-D-ribulose 1-phosphate kcat is 3.5 sec(-1) with 5-deoxy-alpha-D-ribose 1-phosphate as substrate. Carbohydrate degradation. Homodimer. Cells lacking this gene show no impaired growth on minimal medium but addition of 5-deoxyribose inhibits growth at a higher level than wild-type. Moreover, the intracellular level of 5-deoxy-D-ribulose 1-phosphate increases in wild-type cells supplied with 5-deoxyribose, but not in the deletion mutant cells. Belongs to the EIF-2B alpha/beta/delta subunits family. DrdI subfamily.