THE BROAD SUBSTRATE CHLOROBENZENE DIOXYGENASE AND CIS-CHLOROBENZENE DIHYDRODIOL DEHYDROGENASE OF PSEUDOMONAS SP STRAIN P51 ARE LINKED EVOLUTIONARILY TO THE ENZYMES FOR BENZENE AND TOLUENE DEGRADATION

The chlorobenzene degradation pathway of Pseudomonas sp, strain P51 is an evolutionary novelty, The first enzymes of the pathway, the chloro benzene dioxygenase and the cis-chlorobenzene dihydrodiol dehydrogenas e, are encoded on a plasmid-located transposon Tn5280. Chlorobenzene d ioxygenase is a four-protein complex, formed by the gene products of t cbAa for the large subunit of the terminal oxygenase, tcbAb for the sm all subunit, tcbAc for the ferredoxin, and tcbAd for the NADH reductas e, Directly downstream of tcbAd is the gene for the cis-chlorobenzene dihydrodiol dehydrogenase, tcbB, Homology comparisons indicated that t hese genes and gene products are most closely related to those for tol uene (todC1C2BAD) and benzene degradation (bedC1C2BA and bnzABCD) and distantly to those for biphenyl, naphthalene, and benzoate degradation , Similar to the tod-encoded enzymes, chlorobenzene dioxygenase and ci s-chlorobenzene dihydrodiol dehydrogenase were capable of oxidizing 1, 2-dichlorobenzene, toluene, naphthalene, and biphenyl, but not benzoat e, to the corresponding dihydrodiol and dihydroxy intermediates, These data strongly suggest that the chlorobenzene dioxygenase and dehydrog enase originated from a toluene or benzene degradation pathway, probab ly by horizontal gene transfer, This evolutionary event left its trace s as short gene fragments directly outside the tcbAB coding regions.