Major phosphoribomutase that converts ribose 1-phosphate to ribose 5-phosphate. Involved in ribose salvage via the pentose phosphate pathway. alpha-D-ribose 1-phosphate = D-ribose 5-phosphate Binds 1 Mg(2+) ion per subunit. Hyperaccumulates ribose 1-phosphate and upstream purines upon glucose-induced purine nucleoside recycling. Present with 4960 molecules/cell in log phase SD medium. Belongs to the phosphohexose mutase family.