Catalyzes the isomerization of ribose 1,5-bisphosphate (R15P) to ribulose 1,5-bisphosphate (RuBP), the CO(2) acceptor and substrate for RubisCO. Only accepts the alpha-anomer of D-ribose 1,5-bisphosphate as substrate, being inactive on the beta-anomer. Displays a strict substrate specificity, since other phosphorylated sugars such as R5P, ribose, G16P, G6P, G1P, FBP, F6P, and PRPP, are not substrates. Functions in an archaeal AMP degradation pathway, together with AMP phosphorylase and RubisCO. alpha-D-ribose 1,5-bisphosphate = D-ribulose 1,5-bisphosphate Is highly activated in the presence of AMP, with an increase of >40-fold in activity levels. Among other nucleotides, isomerase activity is slightly increased in the presence of GMP, but CMP, UMP, TMP, and NAD(+) have no effect; therefore, AMP is likely the major activator of R15P isomerase in vivo. To a lesser extent, various compounds with an adenosyl moiety, such as dAMP, adenosine, or methylthioadenosine, can also act as activators. The regulation of this enzyme by AMP prevents excess degradation of intracellular AMP by the archaeal AMP degradation pathway. kcat is 29.2 sec(-1) (in the presence of AMP, at 85 degrees Celsius). Homohexamer; trimer of dimers. Up-regulated by nucleosides (at protein level). Reaction proceeds via a cis-phosphoenolate intermediate. Belongs to the eIF-2B alpha/beta/delta subunits family. R15P isomerase subfamily.