Involved in the biosynthesis of K-antigen capsules. Catalyzes the reversible aldol-ketol isomerization between D-ribulose 5-phosphate (Ru5P) and D-arabinose 5-phosphate (A5P) (By similarity). D-arabinose 5-phosphate = D-ribulose 5-phosphate Homotetramer. Belongs to the SIS family. GutQ/KpsF subfamily.