Catalyzes the reversible aldol-ketol isomerization between D-ribulose 5-phosphate (Ru5P) and D-arabinose 5-phosphate (A5P). It is also able of sustaining the biosynthetic pathway of 3-deoxy-D-manno-octulosonate (KDO), a unique 8-carbon sugar component of lipopolysaccharides (LPSs) (By similarity). D-arabinose 5-phosphate = D-ribulose 5-phosphate Homotetramer. Belongs to the SIS family. GutQ/KpsF subfamily. Truncated N-terminus. Truncated N-terminus.