DISTRIBUTION OF THE TFDA GENE IN SOIL BACTERIA THAT DO NOT DEGRADE 2,4-DICHLOROPHENOXYACETIC ACID (2,4-D)

The tfdA gene encodes an alpha-ketoglutarate-dependent dioxygenase tha t catalyzes the initial step in the degradation of the widely used her bicide, 2,4-dichlorophenoxyacetic acetic acid (2,4-D). Since its descr iption in Alcaligenes eutrophus JMP134, tfdA has been found on plasmid s or on the chromesomes of phylogenetically diverse 2,4-D-degrading mi croorganisms. To assess its distribution in more diverse microbial pop ulations, including bacteria that do not degrade 2,4-D, a collection o f 76 soil bacteria that had been isolated on nonselective medium was s creened for the presence of this gene by PCR amplification of an inter nal fragment of tfdA. PCR amplification products of the expected size were generated from the genomic DNA of 37% of these isolates. The prod ucts were shown to be highly similar to previously described tfdA sequ ences, either by hybridization of tfdA gene probes or by DNA sequence analyses. None of the isolates were able to degrade 2,4-D in liquid me dium, or incorporate radiolabeled carbon from ring- or chain-labeled C -14-2,4-D. Characterization of these isolates by analysis of 16S rDNA partial sequence data showed the extensive distribution of tfdA in soi l isolates from the same soil community. Microorganisms containing tfd A were found in many different phylogenetic groups, including alpha, b eta, and gamma Proteobacteria, and both the Bacillus-Lactobacillus-Str eptococcus subdivision and high G+C group of the Gram-positive phylum. These data suggest that tfdA, or closely related homologs, are widesp read among soil isolates and are likely to exist in these organisms fo r a purpose other than the degradation of 2,4-D.