Studies of Degradation of Xenobiotic Compound: A Window on the Evolution of a Novel Metabolic Pathway at an Early Stage of Development

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

This project will address the evolution of the pathway for degradation of pentachlorophenol (PCP) in the soil bacterium Sphingomonas chlorophenolica. Since PCP was introduced only in 1936, this pathway must have evolved and assembled rather recently in microorganisms. This experimental system provides an unusual opportunity to study a metabolic pathway at a very early stage in its development. The emergence of this pathway is also of special interest because PCP is highly toxic, as well as being xenobiotic, and is thus a particularly challenging target for degradation. Previous work has suggested that tetrachlorohydroquinone dehalogenase, the second enzyme in the pathway, may have originated from a glutathione-dependent double bond isomerase such as maleylacetoacetate isomerase or maleylpyruvate isomerase (enzymes involved in degradation of tyrosine and benzoate, respectively). The first objective of the project is to further explore the origin of this enzyme. Levels of dehalogenase activity in several maleylacetoacetate and maleylpyruvate isomerases will be measured in order to evaluate the hypothesis that this enzyme was recruited to serve as a dehalogenase because it fortuitously had a low but useful level of dehalogenase activity. The second objective of the project is to investigate the genetic context of the dehalogenase gene in order to determine whether it originated from a gene duplication event in S. chlorophenolica or was introduced by horizontal transfer from another organism. This project addresses the mechanisms by which soil bacteria evolve new metabolic pathways to degrade anthropogenic pollutants which have been recently introduced into the environment as a result of agricultural, industrial, military, or household uses. Such pollutants are often resistant to biodegradation and consequently persist in the environment. The ability of bacteria to evolve new pathways to degrade anthropogenic compounds is a critical factor that affects the environmental fate of these compounds. This project concerns the evolution of a new metabolic pathway for degradation of pentachlorophenol, a widely used and highly toxic wood preservative, in a soil bacterium called Sphingomonas chlorophenolica. This bacterium has apparently evolved a new pathway by recruiting proteins already present in the cell to perform new functions. The evolutionary origin of one of these enzymes, tetrachlorohydroquinone dehalogenase, will be studied during this project.