Two new genes whose products are involved in biodegradation of the organic solvent tetralin were identified. These genes, designated thnE and thnF, are located downstream of the previously identified thnD gene and code for a hydratase and an aldolase, respectively. A sequence comparison of enzymes similar to ThnE showed the significant similarity of hydratases involved in biodegradation pathways to 4-oxalocrotonate decarboxylases and established four separate groups of related enzymes. Consistent with the sequence information, characterization of the reaction catalyzed by ThnE showed that it hydrated a 10-carbon dicarboxylic acid. The only reaction product detected was the enol tautomer, 2,4-dihydroxydec-2-ene-1,10-dioic acid. The aldolase ThnF showed significant similarity to aldolases involved in different catabolic pathways whose substrates are dihydroxylated dicarboxylic acids and which yield pyruvate and a semialdehyde. The reaction products of the aldol cleavage reaction catalyzed by ThnF were identified as pyruvate and the seven-carbon acid pimelic semialdehyde. ThnF and similar aldolases showed conservation of the active site residues identified by the crystal structure of 2-dehydro-3-deoxy-galactarate aldolase, a class II aldolase with a novel reaction mechanism, suggesting that these similar enzymes are class II aldolases. In contrast, ThnF did not show similarity to 4-hydroxy-2-oxovalerate aldolases of other biodegradation pathways, which are significantly larger and apparently are class I aldolases.The organic solvent tetralin (1,2,3,4-tetrahydronaphthalene) is produced for industrial purposes from naphthalene by catalytic hydrogenation or from anthracene by cracking, and it is widely used as a degreasing agent and solvent for fats, resins, and waxes, as a substitute for turpentine in paints, lacquers, and shoe polishes, and also in the petrochemical industry in connection with coal liquefaction (6). A concentration of tetralin higher than 100 M inhibits bacterial growth (24). The toxicity of this compound is due in part to its lipophilic character, which results in accumulation in the cell membrane, leading to changes in the structure and function of the membrane (26,27). In addition, tetralin also forms toxic hydroperoxides in the cell (5).Tetralin is a bicyclic molecule composed of an aromatic moiety and an alicyclic moiety, which share two carbon atoms. In principle, initial transformation of tetralin may involve metabolism of either the aromatic ring or the alicyclic ring. A few bacterial strains that are able to grow on tetralin as a sole carbon and energy source have been described (24). By identifying accumulated intermediates, several authors have suggested that some bacteria, such as Pseudomonas stutzeri AS39 (23), initially hydroxylate and further oxidize the alicyclic ring, while other bacteria, such as Corynebacterium sp. strain C125 (25), initially dioxygenate the aromatic ring, which is cleaved in the extradiol position (meta-cleavage pathway); thus, the data indicate that tetralin may...