Catalyzes the isomerization of L-erythrulose-1P to D-erythrulose-4P. Involved in the degradation pathway of L-threitol, that allows M.smegmatis to grow on this compound as the sole carbon source. L-erythrulose 1-phosphate = D-erythrulose 4-phosphate Carbohydrate metabolism; L-threitol degradation. Cells lacking this gene are totally unable to grow on L-threitol. Belongs to the triosephosphate isomerase family. The substrate of the reaction was originally identified as L-erythrulose 4-phosphate (PubMed:26560079). It was then corrected to L-erythrulose 1-phosphate by the same group (PubMed:26978037). Extended N-terminus.