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  • Introduction Heme is an important small molecule


    Introduction Heme is an important small molecule and an essential cofactor for a variety of enzymes (George, 1948; Morrison and Stotz, 1954; Maehly, 1952; Igo et al., 1961). During cellular respiration, heme populates cytochromes and serves as an electron acceptor in the electron transport chain (Morrison and Stotz, 1954; Hammer et al., 2013). Heme-dependent respiration is critical for many organisms (Musser and Chan, 1998). If heme is unable to populate the cytochromes, either due to genetic inactivation of the cytochromes or a lack of cellular heme, spectinomycin are unable to respire and must switch to a fermentative state (Von Eiff et al., 1997). Fermentation through glycolysis results in the production of 2 ATP molecules, compared to respiration that can generate up to 38 molecules of ATP per molecule of glucose. Staphylococcus aureus is a Gram-positive coccoid bacterium and is the leading cause of skin and soft tissue infections (Klevens et al., 2007). In order to meet the cellular requirements for heme, S. aureus both biosynthesizes heme and imports heme from the extracellular milieu (Mazmanian et al., 2003; Tien and White, 1968). Heme import is mediated through the high-affinity iron-regulated surface determinant (Isd) heme acquisition system (Mazmanian et al., 2003; Torres et al., 2006). The genes in the isd operons are regulated by the Ferric Uptake Regulator (Fur) (Mazmanian et al., 2003; Xiong et al., 2000). Fur dimerizes when iron is present to bind Fur boxes in the promoter regions of target genes and repress transcription. This repression is alleviated under iron deplete conditions, when there is insufficient intracellular iron to allow Fur dimerization (Bagg and Neilands, 1987). Regulation by Fur is widely conserved throughout bacterial species, and Fur regulates a variety of transcripts associated with pathogenesis (Torres et al., 2010; Litwin et al., 1992; Tsolis et al., 1995; Tanui et al., 2017; Beall and Sanden, 1995). In addition to being an important enzymatic cofactor, heme can also be used as a source of iron. Vertebrate-associated microorganisms, especially pathogenic bacteria, exploit host heme as a nutrient source. Aerobically, heme degradation is performed by heme oxygenases, while anaerobic bacteria use enzymes that rely on radical catabolism (LaMattina et al., 2016). Once heme is imported into the cell through the Isd proteins, heme is used to populate heme-binding proteins or heme is degraded by heme degrading enzymes. S. aureus encodes two such heme degrading enzymes, the heme oxygenases IsdI and IsdG. IsdG and IsdI facilitate the degradation of heme to produce the secondary catabolites staphylobilin and formaldehyde (Reniere et al., 2010; Matsui et al., 2013). This degraded heme also results in the release of iron, which the bacteria can use to meet their iron requirements (Skaar et al., 2004a). Here we describe work initiated to understand the role of the heme degradation products in the context of S. aureus. Through an RNA-Sequencing experiment, comparing a S. aureus strain containing constitutive heme oxygenase activity to one lacking heme degradation, we found a significant increase in a number of transcripts from genes associated with oxygen-independent energy production. This led to the hypothesis that dysregulation of isdI causes aberrant degradation of the intracellular heme pool. Further analysis comparing constitutively and endogenously expressed isdI and isdG showed a significant decrease in intracellular heme levels and heightened fermentation. This work demonstrates the importance of Fur regulation for optimal bacterial growth in low iron conditions.
    Discussion Heme is a cofactor for a variety of cellular processes, including cytochromes where it facilitates respiration by acting as an electron acceptor (Morrison and Stotz, 1954; Hammer et al., 2013; Hurt and Hauska, 1981). If S. aureus is unable to make or acquire heme, then the cell must depend on fermentation for energy production, which generates less ATP than respiration (Jurtshuk, 1996). The production of less energy leads to significantly less growth and to changes in cellular physiology.