Identification of an Extradiol Dioxygenase Involved in Tetralin Biodegradation: Gene Sequence Analysis and Purification and Characterization of the Gene Product

Abstract

A genomic region involved in tetralin biodegradation was recently identified in Sphingomonas strain TFA. We have cloned and sequenced from this region a gene designated thnC, which codes for an extradiol dioxygenase required for tetralin utilization. Comparison to similar sequences allowed us to define a subfamily of 1,2-dihydroxynaphthalene extradiol dioxygenases, which comprises two clearly different groups, and to show that ThnC clusters within group 2 of this subfamily. 1,2-Dihydroxy-5,6,7,8-tetrahydronaphthalene was found to be the metabolite accumulated by a thnC insertion mutant. The ring cleavage product of this metabolite exhibited behavior typical of a hydroxymuconic semialdehyde toward pH-dependent changes and derivatization with ammonium to give a quinoline derivative. The gene product has been purified, and its biochemical properties have been studied. The enzyme is a decamer which requires Fe(II) for activity and shows high activity toward its substrate (V max , 40.5 U mg ؊1 ; K m , 18.6 M). The enzyme shows even higher activity with 1,2-dihydroxynaphthalene and also significant activity toward 1,2-dihydroxybiphenyl or methylated catechols. The broad substrate specificity of ThnC is consistent with that exhibited by other extradiol dioxygenases of the same group within the subfamily of 1,2-dihydroxynaphthalene dioxygenases.Tetralin (1,2,3,4-tetrahydronaphthalene) is an organic solvent 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 (12). Tetralin is very toxic to bacteria because of its accumulation in the cell membranes, which leads to changes in structure and function (39,40), and also because of the formation of highly toxic hydroperoxides (11).Tetralin is a bicyclic molecule composed of an aromatic moiety and an alicyclic moiety sharing two carbon atoms. Just a few bacterial strains able to grow on tetralin as the only carbon and energy source have been reported (37), and very little is known about the utilization of this molecule by bacteria. Identification of accumulated intermediates during growth on tetralin suggests that some bacteria, such as Pseudomonas stutzeri AS39, initially hydroxylate and further oxidize the alicyclic ring (35), while others, such as Corynebacterium sp. strain C125, initially dioxygenate the aromatic ring, which is subsequently cleaved in the extradiol position (38). In spite of previous reports showing modification and utilization of tetralin (35,38,41,42), a complete biodegradation pathway has not yet been elucidated.Key enzymes in the pathways of aromatic compounds are the metal-dependent ring cleavage dioxygenases, which act on the corresponding catechol-type derivatives, cleaving them at the intradiol position (ortho cleavage) or the extradiol position (meta cleavage) (18). While intradiol dioxygenases typically depend on Fe(III), most extradiol dioxygenases depend on Fe(II), although one m...

High aromatic ring-cleavage diversity in birch rhizosphere: PAH treatment-specific changes of I.E.3 group extradiol dioxygenases and 16S rRNA bacterial communities in soil

“…The dominant OTU 72 in Bulk-1200 soil (69 clones in both replicates) was placed in the ThnC cluster named according to the thnC gene from Sphingopyxis macrogoltabida TFA strain. The ThnC enzyme catalyses the ring cleavage of tetralin, an aromatic compound composed of one aromatic ring structure fused to a six-carbon aliphatic ring structure (Andujar et al, 2000).…”

Identification of a Serine Hydrolase Which Cleaves the Alicyclic Ring of Tetralin

“…Physical and genetic analysis of mutants led to the identification of a genomic region involved in tetralin biodegradation which comprises two divergent operons (22). Identification of thnC, coding for an extradiol dioxygenase, and characterization of the reaction catalyzed by its gene product indicated that biodegradation of tetralin by strain TFA also involves an initial metabolization of the aromatic ring by a meta-cleavage catabolic pathway (2). However, a complete biodegradation pathway has not yet been established.…”

“…Catechol, 3-methylcatechol, 4-methylcatechol, and 2,3-dihydroxybiphenyl were purchased from Aldrich at the highest purity available. The corresponding ring fission products were synthesized biologically by whole cells of NCM631/ pIZ227,pIZ591, which overproduce the broad-substrate-specificity extradiol dioxygenase ThnC (2).…”

“…Dihydroxytetralin (DHT) was chemically synthesized (2). Catechol, 3-methylcatechol, 4-methylcatechol, and 2,3-dihydroxybiphenyl were purchased from Aldrich at the highest purity available.…”

“…Plasmids pIZ608, pIZ591, and pIZ578 have been described elsewhere (2,22). A 1,349-bp StuI-ClaI fragment from pIZ608 was cloned into pBluescript II KS(ϩ) (Stratagene) and commercially sequenced (Boehringer Mannheim).…”