Doctoral Dissertation Defense - Jonathan Phan

Microbial Biology Graduate Program

Doctoral Dissertation Defense

Jonathan Phan

"Molecular Mechanisms of Microbial Sulfur and Selenium Reduction"

Committee Members: Dr. Nathan Yee, Dr. Costantino Vetriani, Dr. Lee Kerkhof, Dr. Ileana Pérez-Rodríguez

Dissertation Summary

Microbial reduction of sulfur and selenium compounds controls the speciation of these elements in soils and aquatic systems, governing the reactivity of organic sulfur and the mobility and toxicity of selenium. This dissertation examined the molecular basis of these processes and how readily they transfer between bacteria. Shewanella oneidensis MR-1 was shown to reduce extracellular organic disulfides through the same Mtr pathway it uses to respire insoluble metal oxides, extending the substrate range of extracellular electron transfer to a defined organic functional group. In Enterobacter cloacae SLD1a-1, genome sequencing and targeted mutagenesis identified srnABCD, a twin-arginine translocation (TAT)-exported molybdoenzyme operon encoding the long-sought selenate reductase. Deletion of the catalytic subunit gene srnA abolished selenate reduction, complementation restored it, and heterologous expression in
Escherichia coli conferred activity on a mutant strain that does not reduce selenate. Introducing srnABCD into S. oneidensis MR-1 on a broad-host-range plasmid produced a strain capable of reducing selenate, a capability the wild type lacks. Together, these studies define the molecular basis of two reductive processes and demonstrate that the genetic determinants of selenate reduction remain functional outside their native host, with implications for the fate and bioremediation of selenium in contaminated environments.