title: Aureusimines in Staphylococcus aureus are not involved in virulence.
abstract: BACKGROUND
  Recently, dipeptide aureusimines were reported to activate expression of staphylococcal virulence genes, such as alpha-hemolysin, and increase S. aureus virulence. Surprisingly, most of the virulence genes affected by aureusimines form part of the regulon of the SaeRS two component system (TCS), raising the possibility that SaeRS might be directly or indirectly involved in the aureusimine-dependent signaling process.
  
  
  RESULTS
  Using HPLC analyses, we confirmed that a transposon mutant of ausA, the gene encoding the aureusimine dipeptide synthesis enzyme, does not produce dipeptides. However, the transposon mutant showed normal hemolysis activity and alpha-hemolysin/SaeP production. Furthermore, the P1 promoter of the sae operon, one of the targets of the SaeRS TCS, showed normal transcription activity. Moreover, in contrast to the original report, the ausA transposon mutant did not exhibit attenuated virulence in an animal infection model. DNA sequencing revealed that the ausA deletion mutant used in the original study has an 83 nt-duplication in saeS. Hemolysis activity of the original mutant was restored by a plasmid carrying the sae operon. A mutant of the sae operon showed elevated resistance to chloramphenicol and erythromycin, two antibiotics widely used during staphylococcal mutagenesis. At 43°C in the presence of erythromycin and aeration, the conditions typically employed for staphylococcal mutagenesis, an saeR transposon mutant grew much faster than a control mutant and the saeR mutant was highly enriched in a mixed culture experiment.
  
  
  CONCLUSIONS
  Our results show that the previously reported roles of aureusimines in staphylococcal gene regulation and virulence were due to an unintended mutation in saeS, which was likely selected due to elevated resistance of the mutant to environmental stresses. Thus, there is no evidence indicating that the dipeptide aureusimines play a role in sae-mediated virulence factor production or contribute to staphylococcal virulence.
authors.affiliation: Department of Chemistry and Institute for Biophysical Dynamics, University of Chicago, Chicago, Illinois, United States of America.
authors.forename: Fei
authors.initials: F
authors.lastname: Sun
authors.forename: Hoonsik
authors.initials: H
authors.lastname: Cho
authors.forename: Do-Won
authors.initials: DW
authors.lastname: Jeong
authors.forename: Chunling
authors.initials: C
authors.lastname: Li
authors.forename: Chuan
authors.initials: C
authors.lastname: He
authors.forename: Taeok
authors.initials: T
authors.lastname: Bae
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics*; D001483:Base Sequence; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D017353:Gene Deletion; D015964:Gene Expression Regulation, Bacterial; D008957:Models, Genetic; D008969:Molecular Sequence Data; D009154:Mutation; D010957:Plasmids / Q000378:metabolism; D016133:Polymerase Chain Reaction / Q000379:methods; D011494:Protein Kinases / Q000235:genetics*; D011719:Pyrazines / Q000737:chemistry; D015398:Signal Transduction; D013211:Staphylococcus aureus / Q000378:metabolism*; D014157:Transcription Factors; D014774:Virulence / Q000235:genetics*
chemical_list: D001426:Bacterial Proteins; D011719:Pyrazines; C433805:SaeR protein, Staphylococcus aureus; D014157:Transcription Factors; C551205:aureusimine A; D011494:Protein Kinases; C527351:SaeS protein, Staphylococcus aureus