CorE from Myxococcus xanthus Is a Copper-Dependent RNA Polymerase Sigma Factor

The dual toxicity/essentiality of copper forces cells to maintain a tightly regulated homeostasis for this metal in all living organisms, from bacteria to humans. Consequently, many genes have previously been reported to participate in copper detoxification in bacteria. Myxococcus xanthus, a prokaryote, encodes many proteins involved in copper homeostasis that are differentially regulated by this metal. A σ factor of the ECF (extracytoplasmic function) family, CorE, has been found to regulate the expression of the multicopper oxidase cuoB, the P1B-type ATPases copA and copB, and a gene encoding a protein with a heavy-metal-associated domain. Characterization of CorE has revealed that it requires copper to bind DNA in vitro. Genes regulated by CorE exhibit a characteristic expression profile, with a peak at 2 h after copper addition. Expression rapidly decreases thereafter to basal levels, although the metal is still present in the medium, indicating that the activity of CorE is modulated by a process of activation and inactivation. The use of monovalent and divalent metals to mimic Cu(I) and Cu(II), respectively, and of additives that favor the formation of the two redox states of this metal, has revealed that CorE is activated by Cu(II) and inactivated by Cu(I). The activation/inactivation properties of CorE reside in a Cys-rich domain located at the C terminus of the protein. Point mutations at these residues have allowed the identification of several Cys involved in the activation and inactivation of CorE. Based on these data, along with comparative genomic studies, a new group of ECF σ factors is proposed, which not only clearly differs mechanistically from the other σ factors so far characterized, but also from other metal regulators

Selenite-mediated production of superoxide radical anions in A549 cancer cells is accompanied by a selective increase in SOD1 concentration, enhanced apoptosis and Se-Cu bonding

Selenite may exert its cytotoxic effects against cancer cells via the generation of reactive oxygen species (ROS). We investigated sources of, and the cellular response to, superoxide radical anion (O2 ·−) generated in human A549 lung cancer cells after treatment with selenite. A temporal delay was observed between selenite treatment and increases in O2 ·− production and biomarkers of apoptosis/necrosis, indicating that the reduction of selenite by the glutathione reductase/NADPH system (yielding O2 ·−) is a minor contributor to ROS production under these conditions. By contrast, mitochondrial and NADPH oxidase O2 ·− generation were the major contributors. Treatment with a ROS scavenger [poly(ethylene glycol)-conjugated superoxide dismutase (SOD) or sodium 4,5-dihydroxybenzene-1,3-disulfonate] 20 h after the initial selenite treatment inhibited both ROS generation and apoptosis determined at 24 h. In addition, SOD1 was selectively upregulated and its perinuclear cytoplasmic distribution was colocalised with the cellular distribution of selenium. Interestingly, messenger RNA for manganese superoxide dismutase, catalase, inducible haem oxygenase 1 and glutathione peroxidase either remained unchanged or showed a delayed response to selenite treatment. Colocalisation of Cu and Se in these cells (Weekley et al. in J. Am. Chem. Soc. 133:18272–18279, 2011) potentially results from the formation of a Cu–Se species, as indicated by Cu K-edge extended X-ray absorption fine structure spectra. Overall, SOD1 is upregulated in response to selenite-mediated ROS generation, and this likely leads to an accumulation of toxic hydrogen peroxide that is temporally related to decreased cancer cell viability. Increased expression of SOD1 gene/protein coupled with formation of a Cu–Se species may explain the colocalisation of Cu and Se observed in these cells.Claire M. Weekley, Gloria Jeong, Michael E. Tierney, Farjaneh Hossain, Aung Min Maw, Anu Shanu, Hugh H. Harris, Paul K. Wittin