The mtl operon of Klebsiella pneumoniae KAY2026 (formerly Aerobacter aerogenes 1033-5P14) was shown to contain as the promoterproximal gene mtlA, encoding a D-mannitol-specific enzyme II transporter (IICBA Mtl ). This gene is followed by mtlD, coding for a mannitol-1-phosphate dehydrogenase (MtlD, 382 amino acid residues), and mtlR (MtlR, 195 amino acid residues) coding for a putative repressor, gene mtlR overlaps the termination codon of mtlD. The DNA and protein sequences are highly similar to the corresponding genes (81% identical bp) and proteins (79^85% identical amino acids) of Escherichia coli K-12. A truncated form of MtlD lacking the 162 Cterminal amino acid residues still shows 10% dehydrogenase activity which may explain the controversy in the literature concerning the properties of mannitol-phosphate and other medium-length dehydrogenases. ß