The proposed research is to investigate how plant-associated bacteria adapt to toxic levels of copper in the environment. Copper is an important bactericide used frequently for disease control on agricultural plants, and microbes associated with those plants are therefore exposed periodically to high levels of this metal. The development of copper resistance in plant pathogenic bacteria may pose a threat to the future use of copper in plant disease control, and a better understanding of how this resistance develops will be important for evaluating this threat. In addition, knowledge of how plant microbes develop copper resistance may be useful in designing methods for detoxifying copper in contaminated wastewaters or for genetically modifying plants and beneficial microbes for increased tolerance to copper in the agricultural environment. Microbial resistance to toxic metals such as mercury has been studied extensively, but copper resistance has received much less attention. Unlike many other heavy meals, copper is required by living organisms as a component of certain enzymes; at low levels it is beneficial, but at high levels it is toxic. The evolution of copper resistance in microbial populations must therefore involve a balance between mechanisms to prevent copper toxicity and the ability to maintain enough copper for cell growth. Dr. Cooksey isolated copper-resistant bacteria from an agricultural system where copper compounds are routinely applied to plants for bacterial disease control. Genes encoding copper resistance have been cloned and characterized. This proposal concerns the biochemical function of the cloned genes, their genetic regulation, and their relationship to homologous genes that have recently been detected in several bacterial species. Dr. Cooksey has hypothesized that the copper resistance genes have evolved from genes that produce normal copper-containing proteins. These proteins may have been modified to bind more copper and prevent the toxic metal from reaching high concentrations in solution inside the bacterial cell. Since copper is essential for growth, however, the resistance genes should be expressed only when toxic levels of copper are present; Dr. Cooksey has shown that the resistance genes are copper- inducible, and this proposal addresses basic mechanisms of copper-inducible gene expression.
