The enzyme which cleaves the benzene ring of 6-chlorohydroxyquinol was purified to apparent homogeneity from an extract of 2,4,6-trichlorophenol-grown cells of Streptomyces rochei 303. Like the analogous enzyme from Azotobacter sp. strain GP1, it exhibited a highly restricted substrate specificity and was able to cleave only 6-chlorohydroxyquinol and hydroxyquinol and not catechol, chlorinated catechols, or pyrogallol. No extradiolcleaving activity was observed. In contrast to 6-chlorohydroxyquinol 1,2-dioxygenase from Azotobacter sp. strain GP1, the S. rochei enzyme had a distinct preference for 6-chlorohydroxyquinol over hydroxyquinol (k cat /K m ؍ 1.2 and 0.57 s ؊1 ⅐ M ؊1 , respectively). The enzyme from S. rochei appears to be a dimer of two identical 31-kDa subunits. It is a colored protein and was found to contain 1 mol of iron per mol of enzyme. The NH 2 -terminal amino acid sequences of 6-chlorohydroxyquinol 1,2-dioxygenase from S. rochei 303 and from Azotobacter sp. strain GP1 showed a high degree of similarity.Two pathways for the aerobic degradation of chlorophenols have been described so far: one via chlorocatechols and the other via chlorohydroquinones (2, 9, 10, 15). A predominant catabolic route for the compounds carrying one or two chlorine substituents was shown to be the modified ortho-cleavage pathway (14,18,24). In this pathway, chlorocatechols formed by introduction of a second hydroxy group are subjected to intradiol cleavage. In the case of chlorophenol degradation through the chlorohydroquinone pathway, the introduction of a third hydroxy group leads to the formation of hydroxyquinol or chlorohydroxyquinol (3, 13). Both unsubstituted and chlorinated 1,2,4-trihydroxybenzene were shown to be subject to ortho cleavage in Streptomyces rochei 303 (13). Extracts from cells grown in the presence of 2-chloro-or 2,4-dichlorophenol were found to prefer hydroxyquinol over 6-chlorohydroxyquinol as a ring fission substrate, whereas extracts from cells grown on 2,6-dichloro-or 2,4,6-trichlorophenol showed a preference for the chlorinated substrate (13). These results led us to suggest that two different dioxygenases play a role in the cleavage of the trihydroxylated aromatic ring: 6-chlorohydroxyquinol 1,2-dioxygenase in the ring fission of 6-chlorohydroxyquinol, the proposed intermediate of 2,4,6-trichloro-and 2,6-dichlorophenol degradation, and a second dioxygenase in the cleavage of hydroxyquinol, the proposed intermediate of 2,4-dichloro-and 2-chlorophenol degradation (Fig. 1) (13).In this paper, we describe the purification and characterization of 6-chlorohydroxyquinol 1,2-dioxygenase. Three other hydroxyquinol 1,2-dioxygenases have been purified and characterized so far (21,25,34). One of them takes part in the cleavage of hydroxyquinol, found as an intermediate of 4-hydroxybenzoate degradation in Trichosporon cutaneum (34).The second dioxygenase, from the basidiomycete Phanerochaete chrysosporium, catalyzes a key step in the degradation of vanillate, an intermediate in lignin degradation (25)....