Diversity of 2,4-Dichlorophenoxyacetic Acid (2,4-D)-Degradative Genes and Degrading Bacteria

Abstract

Biodegradation of an anthropogenic herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), was documented in the 1950s, the early age of its usage. The 2,4-D-degradative genes in bacteria have long been studied since the first report was published in the 1980s. The well-known 2,4-D catabolic gene cluster tfdABCDFE on the plasmid pJP4 was originally isolated from Cupriavidus pinatubonensis JMP134 and is found to be widely distributed among the phylum Proteobacteria. 2,4-D catabolic gene cluster in pJP4 possesses some distinguished features for the effective degradation and dissemination, e.g., complete gene set located on one site, duplication of the toxic chlorophenol degradation pathway genes, and its self-transmissibility. The dioxygenase gene, tfdA, that catalyzes the initial step of 2,4-D degradation is a distinctive gene in this system on the point that neither similar nucleotide sequence gene nor similar function enzyme was discovered in the environment. tfdA has been used as a good marker or reporter gene for identifying or detecting the 2,4-D degradation system. However, two decades later, a novel 2,4-D degradation gene system, cad gene cluster, was identified from Bradyrhizobium sp. strain HW13 in the 2000s. This finding and subsequent studies changed our whole view of 2,4-D metabolism as tfd gene cluster had long been believed to be an exclusive system responsible for 2,4-D degradation. The initial dioxygenase gene cadA is completely different from tfdA and is a member of aromatic ring hydroxylation dioxygenase genes. In contrast to tfd gene carriers, it is notable that the first organism carrying cadA was discovered from non-2,4-D-contaminated pristine environment. To date, cad system is known to be widespread in the genera Bradyrhizobium and Sphingomonas and perhaps more diverse organisms. In this chapter, we overview the fundamental features, distribution, and competitiveness of those two representative 2,4-D gene systems based upon genetic and biochemical studies together with culture-independent molecular community analyses.