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InStaphylococcus aureus , the global transcriptional regulator CodY modulates the expression of hundreds of genes in response to the availability of GTP and the branched-chain amino acids isoleucine, leucine, and valine (ILV). CodY DNA-binding activity is high when GTP and ILV are abundant. When GTP and ILV are limited, CodY's affinity for DNA drops, altering expression of CodY-regulated targets. In this work, we investigated the impact of guanine nucleotides (GNs) onS. aureus physiology and CodY activity by constructing aguaA null mutant (ΔguaA strain).De novo biosynthesis of guanine monophosphate is abolished due to theguaA mutation; thus, the mutant cells require exogenous guanosine for growth. We also found that CodY activity was reduced when we knocked outguaA , activating the Agr two-component system and increasing secreted protease activity. Notably, in a rich, complex medium, we detected an increase in alternative sigma factor B activity in the ΔguaA mutant, which results in a 5-fold increase in production of the antioxidant pigment staphyloxanthin. Under biologically relevant flow conditions, ΔguaA cells failed to form robust biofilms when limited for guanine or guanosine. Transcriptome sequencing (RNA-Seq) analysis of theS. aureus transcriptome during growth in guanosine-limited chemostats revealed substantial CodY-dependent and -independent alterations of gene expression profiles. Importantly, these changes increase production of proteases and δ-toxin, suggesting thatS. aureus exhibits a more invasive lifestyle when limited for guanosine. Further, gene products upregulated under GN limitation, including those necessary for lipoic acid biosynthesis and sugar transport, may prove to be useful drug targets for treating Gram-positive infections.IMPORTANCE Staphylococcus aureus infections impose a serious economic burden on health care facilities and patients because of the emergence of strains resistant to last-line antibiotics. Understanding the physiological processes governing fitness and virulence ofS. aureus in response to environmental cues is critical for developing efficient diagnostics and treatments.De novo purine biosynthesis is essential for both fitness and virulence inS. aureus since inhibiting production cripplesS. aureus 's ability to cause infection. Here, we corroborate these findings and show that blocking guanine nucleotide synthesis severely affectsS. aureus fitness by altering metabolic and virulence gene expression. Characterizing pathways and gene products upregulated in response to guanine limitation can aid in the development of novel adjuvant strategies to combatS. aureus infections.

Guanine Limitation Results in CodY-Dependent and -Independent Alteration of Staphylococcus aureus Physiology and Gene Expression