Catalyzes the isomerization of ribose 1,5-bisphosphate (R15P) to ribulose 1,5-bisphosphate (RuBP), the CO(2) acceptor and substrate for RubisCO. Functions in an archaeal AMP degradation pathway, together with AMP phosphorylase and RubisCO. alpha-D-ribose 1,5-bisphosphate = D-ribulose 1,5-bisphosphate Reaction proceeds via a cis-phosphoenolate intermediate. Belongs to the eIF-2B alpha/beta/delta subunits family. R15P isomerase subfamily.