UBC Theses and Dissertations
SbnI is a free serine kinase and heme-sensing regulator required for staphyloferrin B biosynthesis in Staphylococcus aureus Verstraete, Meghan Marie
Staphylococcus aureus is a common member of the human microbiome, but is an opportunistic pathogen that can cause a variety of infections. Critical to the growth and survival of S. aureus during infection is acquisition of iron from the host. However, host iron availability is restricted to effectively suppress microbial growth as a type of innate, nutritional immunity. Mechanisms used by S. aureus to access the host iron pool include lysing erythrocytes to liberate hemoglobin for heme uptake and through the secretion of staphyloferrins, which are iron-chelating siderophores to scavenge iron from the host. The multiplicity of iron uptake systems S. aureus possesses is likely a reflection of the varied host environments that S. aureus can colonize or infect. However, the spatiotemporal regulatory mechanisms by which S. aureus adapts to changing iron availability over the course of infection are ill-defined. SbnI is a heme-dependent regulator of staphyloferrin B (SB) biosynthesis suggested to mediate between iron-uptake modes. In this thesis, study of the structure SbnI revealed homology to a free L-serine kinase, SerK, from Thermococcus kodakarensis. Biochemical assays and characterization of a serC mutant of S. aureus showed that SbnI is an ATP dependent L-serine kinase required for production of the SB precursor O-phospho-L-serine. SbnI kinase activity enables SB biosynthesis in environments where S. aureus catabolism is primarily reliant on amino acids, as in abscesses. Characterization of heme binding by SbnI and heme transfer reactions with IsdI and IsdG, two heme degrading enzymes, were used to construct a model of heme-binding by SbnI for regulating heme-SB uptake. This model is consistent with a modest effect of heme-binding on SbnI kinase activity. Heme transfer rates were measured from ChdC, the terminal enzyme in heme biosynthesis, to SbnI, IsdG, and IsdI to delineate an intracellular network of heme sensing and trafficking proteins that are likely required for regulation and adaptation to the dynamic host environment.
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