The frl (fructoselysine) operon encodes fructoselysine 6-kinase and fructoselysine 6-phosphate deglycase, allowing the conversion of fructoselysine into glucose 6-phosphate and lysine. We now show that a third enzyme encoded by this operon catalyses the metal-dependent reversible interconversion of fructoselysine with its C-3 epimer, psicoselysine. The enzyme can be easily assayed through the formation of tritiated water from [3-3H]fructoselysine. Psicoselysine supports the growth of Escherichia coli, causing the induction of the three enzymes of the frl operon. No growth on fructoselysine or psicoselysine was observed with Tn5 mutants in which the putative transporter (FrlA) or fructoselysine 6-phosphate deglycase (FrlB) had been inactivated, indicating the importance of the frl operon for the metabolism of both substrates. The ability of E. coli to grow on psicoselysine suggests the occurrence of this unusual Amadori compound in Nature.