Angular dioxygenation has been established as the crucial step in dibenzofuran degradation by Brevibacterium sp. strain DPO 1361 (V. Strubel, K. H. Engesser, P. Fischer, and H.-J. Knackmuss, J. Bacteriol. 173:1932Bacteriol. 173: -1937Bacteriol. 173: , 1991. The same strain utilizes biphenyl and fluorene as sole sources of carbon and energy. The fluorene degradation sequence is proposed to be initiated by oxidation of the fluorene methylene group to 9-fluorenol. Cells grown on fluorene exhibit pronounced 9-fluorenol dehydrogenase activity. Angular dioxygenation of the 9-fluorenone thus formed yields 1,10-dihydro-1,10-dihydroxyfluoren-9-one (DDF). A mechanistic model is presented for the subsequent C-C bond cleavage by an NAD+-dependent DDF dehydrogenase, acting on the angular dihydrodiol. This enzyme was purified and characterized as a tetramer of four identical 40-kDa subunits. The following Km values were determined: 13 IuM for DDF and 65 ,uM for 2,3-dihydro-2,3-dihydroxybiphenyl. The enzyme also catalyzes the production of 3-(2'-carboxyphenyl)catechol, which was isolated, and structurally characterized, in the form of the corresponding lactone, 4-hydroxydibenzo-(b,d)-pyran-6-one. Stoichiometry analysis unequivocally demonstrates that angular dioxygenation constitutes the principal pathway in Brevibacterium sp. strain DPO 1361.Polynuclear aromatic hydrocarbons contribute significantly to industrial soil contamination. In this respect, bioremediation has gained increasing importance. A recent review (28) contends that bacterial degradation of polynuclear aromatic hydrocarbons is without exception initiated by dioxygenation in either the 1,2, 2,3, or 3,4 position of the condensed ring system.Various pure bacterial cultures have been reported to utilize fluorene as a sole source of carbon and energy. Fluorene degradation via initial 3,4 dioxygenation has been suggested for Arthrobacter sp. strain F101. Since this organism utilizes neither 9-fluorenol nor 9-fluorenone, a pathway via oxidation of the methylene bridge seems unlikely (14). There is some indication that degradation of fluorene by a Pseudomonas vesicularis strain is also initiated by 3,4 dioxygenation (37, 38). For a Staphylococcus auriculans strain reported to grow with dibenzofuran (DBF) and fluorene as sole sources of carbon and energy (18), 1,10-dihydro-1,10-dihydroxyfluoren-9-one (DDF) has been isolated from the culture fluid as one of several metabolites. Since the strain seems to lack the potential to further metabolize this secondary substrate, the authors postulate that degradation is initiated by 3,4 dioxygenation (18).The present report constitutes, to the best of our knowledge, the first in-depth analysis of a bacterial catabolic pathway for fluorene. Brevibacterium sp. strain DPO 1361 was enriched with DBF as the sole source of carbon and energy. The crucial degradation step for this substrate is initial angular dioxygenation (7,9,11,12) transformed into a (chemically unstable) hemiacetal. Thus, 3-(2'-hydroxyphenyl)catechol represents t...