title: Cysteine Addition Promotes Sulfide Production and 4-Fold Hg(II)-S Coordination in Actively Metabolizing Escherichia coli.
abstract: The bacterial uptake of mercury(II), Hg(II), is believed to be energy-dependent and is enhanced by cysteine in diverse species of bacteria under aerobic and anaerobic conditions. To gain insight into this Hg(II) biouptake pathway, we have employed X-ray absorption spectroscopy (XAS) to investigate the relationship between exogenous cysteine, cellular metabolism, cellular localization, and Hg(II) coordination in aerobically respiring Escherichia coli (E. coli). We show that cells harvested in exponential growth phase consistently display mixtures of 2-fold and 4-fold Hg(II) coordination to sulfur (Hg-S2and Hg-S4), with added cysteine enhancing Hg-S4formation. In contrast, cells in stationary growth phase or cells treated with a protonophore causing a decrease in cellular ATP predominantly contain Hg-S2, regardless of cysteine addition. Our XAS results favor metacinnabar (β-HgS) as the Hg-S4species, which we show is associated with both the cell envelope and cytoplasm. Additionally, we observe that added cysteine abiotically oxidizes to cystine and exponentially growing E. coli degrade high cysteine concentrations (100-1000 μM) into sulfide. Thermodynamic calculations confirm that cysteine-induced sulfide biosynthesis can promote the formation of dissolved and particulate Hg(II)-sulfide species. This report reveals new complexities arising in Hg(II) bioassays with cysteine and emphasizes the need for considering changes in chemical speciation as well as growth stage.
authors.affiliation: Department of Civil and Environmental Engineering, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Sara A
authors.identifier: 0000-0003-4970-4431
authors.initials: SA
authors.lastname: Thomas
authors.affiliation: Department of Civil and Environmental Engineering, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Jean-François
authors.identifier: 0000-0002-8276-6418
authors.initials: JF
authors.lastname: Gaillard
mesh_terms: D003545:Cysteine / Q000378:metabolism / Q000494:pharmacology; D004926:Escherichia coli / Q000378:metabolism*; D008628:Mercury / Q000737:chemistry; D013440:Sulfides / Q000737:chemistry / Q000378:metabolism*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D013440:Sulfides; D008628:Mercury; D003545:Cysteine