De novo
guanine biosynthesis is an evolutionarily conserved pathway that creates sufficient
nucleotides to support DNA replication, transcription, and translation. Bacteria can also salvage nutrients from the environment to supplement the de novo pathway, but the relative importance of either pathway during
Staphylococcus aureus infection is not known. In S. aureus, genes important for both de novo and salvage pathways are regulated by a
guanine riboswitch. Bacterial
riboswitches have attracted attention as a novel class of antibacterial drug targets because they have high affinity for small molecules, are absent in humans, and regulate the expression of multiple genes, including those essential for cell viability. Genetic and biophysical methods confirm the existence of a bona fide
guanine riboswitch upstream of an operon encoding
xanthine phosphoribosyltransferase (xpt),
xanthine permease (pbuX), inosine-5'-monophosphate
dehydrogenase (guaB), and
GMP synthetase (guaA) that represses the expression of these genes in response to
guanine. We found that S. aureus guaB and guaA are also transcribed independently of
riboswitch control by alternative promoter elements. Deletion of xpt-pbuX-guaB-guaA genes resulted in
guanine auxotrophy, failure to grow in human serum, profound abnormalities in cell morphology, and avirulence in mouse
infection models, whereas deletion of the
purine salvage genes xpt-pbuX had none of these effects. Disruption of guaB or guaA recapitulates the xpt-pbuX-guaB-guaA deletion in vivo In total, the data demonstrate that targeting the
guanine riboswitch alone is insufficient to treat S. aureus
infections but that inhibition of guaA or guaB could have therapeutic utility.
IMPORTANCE: De novo
guanine biosynthesis and
purine salvage genes were reported to be regulated by a
guanine riboswitch in Staphylococcus aureus We demonstrate here that this is not true, because alternative promoter elements that uncouple the de novo pathway from
riboswitch regulation were identified. We found that in animal models of
infection, the
purine salvage pathway is insufficient for S. aureus survival in the absence of de novo
guanine biosynthesis. These data suggest targeting the de novo
guanine biosynthesis pathway may have therapeutic utility in the treatment of S. aureus
infections.