The 4-chloro-and 2,4-dichlorophenol-degrading strain Rhodococcus opacus 1CP has previously been shown to acquire, during prolonged adaptation, the ability to mineralize 2-chlorophenol. In addition, homogeneous chlorocatechol 1,2-dioxygenase from 2-chlorophenol-grown biomass has shown relatively high activity towards 3-chlorocatechol. Based on sequences of the N terminus and tryptic peptides of this enzyme, degenerate PCR primers were now designed and used for cloning of the respective gene from genomic DNA of strain 1CP. A 9.5-kb fragment containing nine open reading frames was obtained on pROP1. Besides other genes, a gene cluster consisting of four chlorocatechol catabolic genes was identified. As judged by sequence similarity and correspondence of predicted N termini with those of purified enzymes, the open reading frames correspond to genes for a second chlorocatechol 1,2-dioxygenase (ClcA2), a second chloromuconate cycloisomerase (ClcB2), a second dienelactone hydrolase (ClcD2), and a muconolactone isomerase-related enzyme (ClcF). All enzymes of this new cluster are only distantly related to the known chlorocatechol enzymes and appear to represent new evolutionary lines of these activities. UV overlay spectra as well as high-pressure liquid chromatography analyses confirmed that 2-chloro-cis,cis-muconate is transformed by ClcB2 to 5-chloromuconolactone, which during turnover by ClcF gives cis-dienelactone as the sole product. cis-Dienelactone was further hydrolyzed by ClcD2 to maleylacetate. ClcF, despite its sequence similarity to muconolactone isomerases, no longer showed muconolactone-isomerizing activity and thus represents an enzyme dedicated to its new function as a 5-chloromuconolactone dehalogenase. Thus, during 3-chlorocatechol degradation by R. opacus 1CP, dechlorination is catalyzed by a muconolactone isomerase-related enzyme rather than by a specialized chloromuconate cycloisomerase.

The genes responsible for the degradation of 2,4-dichlorophenoxyacetate (2,4-D) by alpha-Proteobacteria have previously been difficult to detect by using gene probes or polymerase chain reaction (PCR) primers. PCR products of the chlorocatechol 1,2-dioxygenase gene, tfdC, now allowed cloning of two chlorocatechol gene clusters from the Sphingomonas sp. strain TFD44. Sequence characterization showed that the first cluster, tfdD,RFCE, comprises all the genes necessary for the conversion of 3,5-dichlorocatechol to 3-oxoadipate, including a presumed regulatory gene, tfdR, of the LysR-type family. The second gene cluster, tfdC2E2F2, is incomplete and appears to lack a chloromuconate cycloisomerase gene and a regulatory gene. Purification and N-terminal sequencing of selected enzymes suggests that at least representatives of both gene clusters (TfdD of cluster 1 and TfdC2 of cluster 2) are induced during the growth of strain TFD44 with 2,4-D. A mutant constructed to contain an insertion in the chloromuconate cycloisomerase gene tfdD still was able to grow with 2,4-D, but more slowly and with a longer lag phase. This, and the detection of additional activity peaks during protein purification suggest that strain TFD44 harbors at least another chloromuconate cycloisomerase gene. The sequence of the tfdCE region was almost identical to that of a partially characterized chlorocatechol catabolic gene cluster of Sphingomonas herbicidovorans MH, whereas the sequence of the tfdC2E2F2 cluster was different. The similarity of the predicted proteins of the tfdD,RFCE and tfdC2E2F2 clusters to known sequences of other Proteobacteria in the database ranged from 42 to 61% identical positions for the first cluster and from 45.5 to 58% identical positions for the second cluster. Between both clusters, the similarities of their predicted proteins ranged from 44.5 to 64% identical positions. Thus, both clusters (together with those of S. herbicidovorans MH) represent deep-branching lines in the respective dendrograms, and the sequence information will help future primer design for the detection of corresponding genes in the environment.

A co-crystallized benzoate-like molecule is also found bound to the metal center forming a distinctive hydrogen bond network as observed previously also in 4-chlorocatechol 1,2-dioxygenase from Rhodococcus opacus 1CP. This is the first structure of an intradiol dioxygenase specialized in hydroxyquinol ring cleavage to be investigated in detail.

A gene cluster containing a gene for maleylacetate reductase (EC 1.3.1.32) was cloned from Ralstonia eutropha 335 T (DSM 531 T ), which is able to utilize 4-fluorobenzoate as sole carbon source. Sequencing of this gene cluster showed that the R. eutropha 335 T maleylacetate reductase gene, macA, is part of a novel gene cluster, which is not related to the known maleylacetate-reductase-encoding gene clusters. It otherwise comprises a gene for a hypothetical membrane transport protein, macB, possibly co-transcribed with macA, and a presumed regulatory gene, macR, which is divergently transcribed from macBA. MacA was found to be most closely related to TftE, the maleylacetate reductase from Burkholderia cepacia AC1100 (62 % identical positions) and to a presumed maleylacetate reductase from a dinitrotoluene catabolic gene cluster from B. cepacia R34 (61 % identical positions). By expressing macA in Escherichia coli, it was confirmed that macA encodes a functional maleylacetate reductase. Purification of maleylacetate reductase from 4-fluorobenzoate-grown R. eutropha 335 T cells allowed determination of the N-terminal sequence of the purified protein, which was shown to be identical to that predicted from the cloned macA gene, thus proving that the gene is, in fact, recruited for growth of R. eutropha 335 T with this substrate. INTRODUCTIONMaleylacetate reductases (EC 1.3.1.32) play a crucial role in the aerobic microbial degradation of aromatic compounds. They catalyse the NADH-or NADPH-dependent reduction of maleylacetate or 2-chloromaleylacetate to 3-oxoadipate ( Fig. 1) or of substituted maleylacetates to substituted 3-oxoadipates. In fungi, maleylacetate reductases contribute to the catabolism of very common substrates, such as tyrosine, gentisate, benzoate, 4-hydroxybenzoate, protocatechuate, vanillate, resorcinol and phenol (Karasevich & Ivoilov, 1977;Buswell & Eriksson, 1979;Gaal & Neujahr, 1979;Sparnins et al., 1979;Anderson & Dagley, 1980;Jones et al., 1995). For bacteria, it has been shown that maleylacetate reductases are involved in the degradation of resorcinol and 2,4-dihydroxybenzoate via hydroxyquinol (Larway & Evans, 1965;Chapman & Ribbons, 1976;Stolz & Knackmuss, 1993). Other substrates whose catabolic routes comprise this activity have a more complex structure, such as 2-hydroxydibenzo-p-dioxin and 3-hydroxydibenzofuran (Armengaud et al., 1999), or carry unusual substituents, such as nitro or sulfo groups (Spain & Gibson, 1991;Feigel & Knackmuss, 1993;Jain et al., 1994;Rani & Lalithakumari, 1994), fluorine or chlorine atoms (Duxbury et al., 1970; Schlömann et al., 1990a;Latus et al., 1995;Zaborina et al., 1995;Daubaras et al., 1996;Miyauchi et al., 1999).The maleylacetate reductase genes characterized so far tend to belong to specialized gene clusters for the degradation of aromatic compounds. Thus, 3-hydroxydibenzofuran and 2-hydroxydibenzo-p-dioxin degradation via hydroxyquinol appears to be encoded by a specialized gene cluster comprising most of the genes for the pathway including a maleylac...

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