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1. Expression and regulation of the mer operon in Thermus thermophilus

   https://doi.org/10.1111/1462-2920.14953  

   Norambuena, Javiera ; Miller, Maximilian ; Boyd, Jeffrey M. ; Barkay, Tamar ( March 2020 , Environmental Microbiology)

   Mercury (Hg) is a highly toxic and widely distributed heavy metal, which some Bacteria and Archaea detoxify by the reduction of ionic Hg (Hg[II]) to the elemental volatile form, Hg(0). This activity is specified by themeroperon. Themeroperon of the deeply branching thermophileThermus thermophilusHB27 encodes for, anO‐acetyl‐l‐homoacetylserine sulfhydrylase (Oah2), a transcriptional regulator (MerR), a hypothetical protein (hp) and a mercuric reductase (MerA). Here, we show that this operon has two convergently expressed and differentially regulated promoters. An upstream promoter, P_oah, controls the constitutive transcription of the entire operon and a second promoter (P_mer), located withinmerR, is responsive to Hg(II). In the absence of Hg(II), the transcription ofmerAis basal and when Hg(II) is present,merAtranscription is induced. This response to Hg(II) is controlled by MerR and genetic evidence suggests that MerR acts as a repressor and activator of P_mer. When the wholemerR, including P_mer, is removed,merAis transcribed from P_oahindependently of Hg(II). These results suggest that the transcriptional regulation ofmerinT.thermophilusis both similar to, and different from, the well‐documented regulation of proteobacterialmersystems, possibly representing an early step in the evolution ofmer‐operon regulation.

    

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2. Physical and Biogeochemical Controls on pH Dynamics in the Northern Gulf of Mexico During Summer Hypoxia

   https://doi.org/10.1029/2019JC015140  

   Jiang, Zong‐Pei ; Cai, Wei‐Jun ; Chen, Baoshan ; Wang, Kui ; Han, Chenhua ; Roberts, Brian J. ; Hussain, Najid ; Li, Qian ( August 2019 , Journal of Geophysical Research: Oceans)

   High‐accuracy spectrophotometric pH measurements were taken during a summer cruise to study the pH dynamics and its controlling mechanisms in the northern Gulf of Mexico in hypoxia season. Using the recently available dissociation constants of the purified m‐cresol purple (Douglas & Byrne, 2017,https://doi.org/10.1016/j.marchem.2017.10.001; Müller & Rehder, 2018,https://doi.org/10.3389/fmars.2018.00177), spectrophotometrically measured pH showed excellent agreement with pH calculated from dissolved inorganic carbon (DIC) and total alkalinity over a wide salinity range of 0 to 36.9 (0.005 ± 0.016,n= 550). The coupled changes in DIC, oxygen, and nutrients suggest that biological production of organic matter in surface water and the subsequent aerobic respiration in subsurface was the dominant factor regulating pH variability in the nGOM in summer. The highest pH values were observed, together with the maximal biological uptake of DIC and nutrients, at intermediate salinities in the Mississippi and Atchafalaya plumes where light and nutrient conditions were favorable for phytoplankton growth. The lowest pH values (down to 7.59) were observed along with the highest concentrations of DIC and apparent oxygen utilization in hypoxic bottom waters. The nonconservative pH changes in both surface and bottom waters correlated well with the biologically induced changes in DIC, that is, per 100‐μmol/kg biological removal/addition of DIC resulted in 0.21 unit increase/decrease in pH. Coastal bottom water with lower pH buffering capacity is more susceptible to acidification from anthropogenic CO₂invasion but reduction in eutrophication may offset some of the increased susceptibility to acidification.

    

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3. Characterization and effect of metal ions on the formation of the Thermus thermophilus Sco mixed disulfide intermediate

   https://doi.org/10.1002/pro.3502  

   Lopez, Liezelle C. ; Mukhitov, Nikita ; Handley, Lindsey D. ; Hamme, Cristina S. ; Hofman, Cristina R. ; Euers, Lindsay ; McKinney, Jennifer R. ; Piers, Amani D. ; Wadler, Ellen ; Hunsicker‐Wang, Laura M. ( October 2018 , Protein Science)

   The Sco protein fromThermus thermophilushas previously been shown to perform a disulfide bond reduction in the Cu_Aprotein fromT. thermophilus, which is a soluble protein engineered from subunit II of cytochromeba₃oxidase that lacks the transmembrane helix. The native cysteines onTtSco andTtCu_Awere mutated to serine residues to probe the reactivities of the individual cysteines. Conjugation of TNB to the remaining cysteine inTtCu_Aand subsequent release upon incubation with the complementaryTtSco protein demonstrated the formation of the mixed disulfide intermediate. The cysteine ofTtSco that attacks the disulfide bond in the targetTtCu_Aprotein was determined to beTtSco Cysteine 49. This cysteine is likely more reactive than Cysteine 53 due to a higher degree of solvent exposure. Removal of the metal binding histidine, His 139, does not change MDI formation. However, altering the arginine adjacent to the reactive cysteine in Sco (Arginine 48) does alter the formation of the MDI. Binding of Cu²⁺or Cu⁺toTtSco prior to reaction withTtCu_Awas found to preclude formation of the mixed disulfide intermediate. These results shed light on a mechanism of disulfide bond reduction by theTtSco protein and may point to a possible role of metal binding in regulating the activity.

   The function of Sco is at the center of many studies. The disulfide bond reduction in Cu_Aby Sco is investigated herein and the effect of metal ions on the ability to reduce and form a mixed disulfide intermediate are also probed.

    

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4. Zero-Valent iron nanoparticles induce reactive oxygen species in the cyanobacterium, Fremyella diplosiphon.

   Gichuki, S. M. ( November 2021 , ACS omega)

   Dr. Krishna Ganesh (Ed.)

   Nanoscale zero-valent iron nanoparticles (nZVIs) are known to boost biomass production and lipid yield in Fremyella diplosiphon, a model biodiesel-producing cyanobacterium. However, the impact of nZVI-induced reactive oxygen species (ROS) in F. diplosiphon has not been evaluated. In the present study, ROS in F. diplosiphon strains (B481-WT and B481-SD) generated in response to nZVI-induced oxidative stress were quantified and the enzymatic response determined. Lipid peroxidation as a measure of oxidative stress revealed significantly higher malondialdehyde content (p < 0.01) in both strains treated with 3.2, 12.8, and 51.2 mg L–1 nZVIs compared to untreated control. In addition, ROS in all nZVI-treated cultures treated with 1.6–25.6 mg L–1 nZVIs was significantly higher than the untreated control as determined by the 2′,7′-dichlorodihydrofluorescein diacetate fluorometric probe. Immunodetection using densitometric analysis of iron superoxide dismutase (SOD) revealed significantly higher SOD levels in both strains treated with nZVIs at 51.2 mg L–1. In addition, we observed significantly higher (p < 0.001) SOD levels in the B481-SD strain treated with 6.4 mg L−1 nZVIs compared to 3.2 mg L–1 nZVIs. Validation using transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy (EDS) revealed adsorption of nZVIs with a strong iron peak in both B481-WT and B481-SD strains. While the EDS spectra showed strong signals for iron at 4 and 12 days after treatment, a significant decrease in peak intensity was observed at 20 days. Future efforts will be aimed at studying transduction mechanisms that cause metabolic and epigenetic alterations in response to nZVIs in F. diplosiphon. 

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5. Mercury methylation by metabolically versatile and cosmopolitan marine bacteria

   https://doi.org/10.1038/s41396-020-00889-4  

   Lin, Heyu ; Ascher, David B. ; Myung, Yoochan ; Lamborg, Carl H. ; Hallam, Steven J. ; Gionfriddo, Caitlin M. ; Holt, Kathryn E. ; Moreau, John W. ( June 2021 , The ISME Journal)

   Abstract Microbes transform aqueous mercury (Hg) into methylmercury (MeHg), a potent neurotoxin that accumulates in terrestrial and marine food webs, with potential impacts on human health. This process requires the gene pair hgcAB , which encodes for proteins that actuate Hg methylation, and has been well described for anoxic environments. However, recent studies report potential MeHg formation in suboxic seawater, although the microorganisms involved remain poorly understood. In this study, we conducted large-scale multi-omic analyses to search for putative microbial Hg methylators along defined redox gradients in Saanich Inlet, British Columbia, a model natural ecosystem with previously measured Hg and MeHg concentration profiles. Analysis of gene expression profiles along the redoxcline identified several putative Hg methylating microbial groups, including Calditrichaeota, SAR324 and Marinimicrobia, with the last the most active based on hgc transcription levels. Marinimicrobia hgc genes were identified from multiple publicly available marine metagenomes, consistent with a potential key role in marine Hg methylation. Computational homology modelling predicts that Marinimicrobia HgcAB proteins contain the highly conserved amino acid sites and folding structures required for functional Hg methylation. Furthermore, a number of terminal oxidases from aerobic respiratory chains were associated with several putative novel Hg methylators. Our findings thus reveal potential novel marine Hg-methylating microorganisms with a greater oxygen tolerance and broader habitat range than previously recognized. 

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