Expression of tfdA genes in aquatic microbial communities during acclimation to 2,4-dichlorophenoxyacetic acid

The role of gene expression during acclimation of aquatic microbial communities was examined by relating transcription of tfdA to the degradation of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). The tfdA gene encodes for a 2,4-D/2-oxoglutarate dioxygenase that transforms 2,4-D to 2,4-dichlorophenol. Transcription of tfdA, the abundance of tfdA genes and 2,4-D degrading populations, and the rate of 2,4-D disappearance were followed in laboratory incubations of two pond water samples that were exposed to 0.11 mM 2,4-D. Both communities responded to 2,4-D exposure by induction of tfdA transcription but the dynamics of transcript abundance and the homology to the tfdA riboprobe suggested different populations of 2,4-D degraders in the two ponds. In one community, where tfdA transcripts were highly homologous to the tfdA gene of Ralstonia eutropha JMP134, transcription of tfdA was transient and dropped while 2,4-D degradation continued. In the other freshwater community, where tfdA genes with a lower similarity to the tfdA gene of strain JMP134 were transcribed, transcript levels remained high although 2,4-D degradation had ceased. Restriction fragment length polymorphism analysis of tfdA amplicons similarly demonstrated the presence of different tfdA loci in the two freshwater communities, and this difference in populations of tfdA genes probably explains the observed difference in dynamics of catabolic gene transcription.