Phytoremediation by Reduction of Toxic Metal Ions

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9513347 Meagher Heavy metal contamination has reached toxic levels in many parts of the world and has become an urgent problem. Our long term goal is to identify and utilize single gene traits to effect ecologically sound solutions to metal ion contamination based on phytoremediation. The project of the bacterial merA gene, mercuric ion reductase (MerA), catalzes the detoxification of ionic mercury, Hg++, reducing it to its less toxic metallic form, Hg0. Recently it has been shown that a highly mutagenized merA gene, merApe9, confers remarkable levels of resistance to Hg++ in transgenic Arabidopsis plants. Transgenic seeds germinate, seedlings grow, and plants flower and set seed on media containing up to 100 uM Hg++ (20 ppm), while control seedlings die during or shortly after germination at 25 uM. These plants evolved significant levels of Hgo in the process. The most resistant plant lines grow as well on Hg++ as unchallenged control plants and acturally require mercury for optimum growth. The transgenic plants also show resistance to toxic levels of Au+++ in media. The immediate goal of this project is to explore metal ion reduction as a method of phytoremediation for a variety of heavy metal pollutants. Preliminary evidence is presented to suggest that besides Hg++ merA pe9 also can reduce Au+++ and Cu++ in E. coli. MerA belongs to a diverse family of sequence-related reductases, which recognize a wide variety of organic and thiol substrates in addition to Hg++. Besides Hg++ MerA already binds Ag+, Cu++, Cd++, and Au+++. Therefore, it is hypothesized that: the merA gene can be mutated to direct the efficient reduction of a wide variety of metal ions for remediation or extraction. The specific aim of the proposal is to generate merA mutants with novel substrate specificities (e.g., enhanced reduction of Hg++ and novel reduction of Cu++, Pb++, and Cd++). In the future transgenic constructs will be examined in model plants for their potential use in the phytoremediation of metal ion pollution.