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 that catalyzes the initial step in the degradation of the widely used herbicide, 2,4-dichlorophenoxyacetic acetic acid (2,4-D). Since its description in Alcaligenes eutrophus JMP134, tfdA has been found on plasmids or on the chromesomes of phylogenetically diverse 2,4-D-degrading microorganisms. To assess its distribution in more diverse microbial populations, including bacteria that do not degrade 2,4-D, a collection of 76 soil bacteria that had been isolated on nonselective medium was screened for the presence of this gene by PCR amplification of an internal fragment of tfdA. PCR amplification products of the expected size were generated from the genomic DNA of 37% of these isolates. The products were shown to be highly similar to previously described tfdA sequences, 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 medium, 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 soil isolates from the same soil community. Microorganisms containing tfdA were found in many different phylogenetic groups, including alpha, beta, and gamma Proteobacteria, and both the Bacillus-Lactobacillus-Streptococcus subdivision and high G+C group of the Gram-positive phylum. These data suggest that tfdA, or closely related homologs, are widespread among soil isolates and are likely to exist in these organisms for a purpose other than the degradation of 2,4-D.