Catalyzes the isomerization of 2-oxo-tetronate to 3-oxo-tetronate. 2-dehydro-L-erythronate = 3-dehydro-L-erythronate 2-dehydro-D-erythronate = 3-dehydro-D-erythronate Deletion decreases growth with L-threonate or D-erythronate as carbon source. Deletion of both otnI and hyi abolishes growth with L-threonate or D-erythronate. Belongs to the hyi family. OtnI subfamily.