Evidence for Interspecies Gene Transfer in the Evolution of 2,4-Dichlorophenoxyacetic Acid Degraders

Abstract

ABSTRACT Small-subunit ribosomal DNA (SSU rDNA) from 20 phenotypically distinct strains of 2,4-dichlorophenoxyacetic acid (2,4-D)-degrading bacteria was partially sequenced, yielding 18 unique strains belonging to members of the alpha, beta, and gamma subgroups of the class Proteobacteria . To understand the origin of 2,4-D degradation in this diverse collection, the first gene in the 2,4-D pathway, tfdA , was sequenced. The sequences fell into three unique classes found in various members of the beta and gamma subgroups of Proteobacteria . None of the α-Proteobacteria yielded tfdA PCR products. A comparison of the dendrogram of the tfdA genes with that of the SSU rDNA genes demonstrated incongruency in phylogenies, and hence 2,4-D degradation must have originated from gene transfer between species. Only those strains with tfdA sequences highly similar to the tfdA sequence of strain JMP134 ( tfdA class I) transferred all the 2,4-D genes and conferred the 2,4-D degradation phenotype to a Burkholderia cepacia recipient.