Microbial Degradation of Toxic Chemicals: Evolutionary Insights

Abstract

Massive amounts of chlorinated compounds have been released in the environment in the form of herbicides/pesticides, solvents, degreasers and as industrially useful compounds, creating major problems of environmental pollution. Bioremediation, which employs microorganisms or their products for enhanced degradation and detoxification of such compounds, is an emerging technology to combat problems of environmental pollution. Unfortunately, most chlorinated compounds are synthetic and are only slowly degraded in nature. One reason for such slow rates of degradation is a lack of evolution in natural microflora of appropriate genes for the complete degradation of such compounds. Using catechol and chlorocatechol as model systems, we have examined the organization and mode of regulation of catechol (cat) and chlorocatechol (clc) degradative genes in a number of microorganisms. Where examined, the clc genes are clustered and appear to have evolved as part of a plasmid with a positive regulatory gene that controls the expression of the entire cluster. The cat genes appear to be chromosomal and regulated by a similar positive regulator which shows appreciable homology with the regulator of the clc genes. These regulators belong to the LysR-type regulator family and are capable of cross-binding with each other’s promoters, allowing cross-activation in some cases. The mode of evolution of regulatory genes that control the expression of structural genes specifying degradation of such toxic chemicals as phenols and chlorocatechols will be described.