Characterization of a Metal-Resistant Bacillus Strain With a High Molybdate Affinity ModA From Contaminated Sediments at the Oak Ridge Reservation

Xiaoxuan Ge; Michael P Thorgersen; Farris L Poole 2nd; Adam M Deutschbauer; John-Marc Chandonia; Pavel S Novichkov; Sara Gushgari-Doyle; Lauren M Lui; Torben Nielsen; Romy Chakraborty; Paul D Adams; Adam P Arkin; Terry C Hazen; Michael W W Adams

doi:10.3389/fmicb.2020.587127

Characterization of a Metal-Resistant Bacillus Strain With a High Molybdate Affinity ModA From Contaminated Sediments at the Oak Ridge Reservation

Front Microbiol. 2020 Oct 19:11:587127. doi: 10.3389/fmicb.2020.587127. eCollection 2020.

Authors

Affiliations

¹ Department of Biochemistry and Molecular Biology, University of Georgia, Athens, GA, United States.
² Environmental Genomics and Systems Biology Division, Lawrence Berkeley National Laboratory, Berkeley, CA, United States.
³ Earth and Environmental Sciences, Lawrence Berkeley National Laboratory, Berkeley, CA, United States.
⁴ Molecular Biosciences and Integrated Bioimaging, Lawrence Berkeley National Laboratory, Berkeley, CA, United States.
⁵ Department of Bioengineering, University of California, Berkeley, Berkeley, CA, United States.
⁶ Department of Civil and Environmental Engineering, The University of Tennessee, Knoxville, Knoxville, TN, United States.

Abstract

A nitrate- and metal-contaminated site at the Oak Ridge Reservation (ORR) was previously shown to contain the metal molybdenum (Mo) at picomolar concentrations. This potentially limits microbial nitrate reduction, as Mo is required by the enzyme nitrate reductase, which catalyzes the first step of nitrate removal. Enrichment for anaerobic nitrate-reducing microbes from contaminated sediment at the ORR yielded Bacillus strain EB106-08-02-XG196. This bacterium grows in the presence of multiple metals (Cd, Ni, Cu, Co, Mn, and U) but also exhibits better growth compared to control strains, including Pseudomonas fluorescens N2E2 isolated from a pristine ORR environment under low molybdate concentrations (<1 nM). Molybdate is taken up by the molybdate binding protein, ModA, of the molybdate ATP-binding cassette transporter. ModA of XG196 is phylogenetically distinct from those of other characterized ModA proteins. The genes encoding ModA from XG196, P. fluorescens N2E2 and Escherichia coli K12 were expressed in E. coli and the recombinant proteins were purified. Isothermal titration calorimetry analysis showed that XG196 ModA has a higher affinity for molybdate than other ModA proteins with a molybdate binding constant (K _D ) of 2.2 nM, about one order of magnitude lower than those of P. fluorescens N2E2 (27.0 nM) and E. coli K12 (25.0 nM). XG196 ModA also showed a fivefold higher affinity for molybdate than for tungstate (11 nM), whereas the ModA proteins from P. fluorescens N2E2 [K _D (Mo) 27.0 nM, K _D (W) 26.7 nM] and E. coli K12[(K _D (Mo) 25.0 nM, K _D (W) 23.8 nM] had similar affinities for the two oxyanions. We propose that high molybdate affinity coupled with resistance to multiple metals gives strain XG196 a competitive advantage in Mo-limited environments contaminated with high concentrations of metals and nitrate, as found at ORR.

Keywords: Bacillus sp. XG196; molybdate transport; molybdenum limitation; nitrate; nitrate reductase.