title: senX3-independent contribution of regX3 to Mycobacterium tuberculosis virulence.
abstract: BACKGROUND
  Mycobacterium tuberculosis (Mtb) must adapt to various stress conditions during host infection. The two-component regulatory system (2CRS) SenX3-RegX3 is required for Mtb virulence. We showed recently that the senX3-regX3 intergenic region contains promoter activity, driving senX3-independent regX3 expression. In the current study, we tested the hypothesis that RegX3 has a SenX3-independent role in Mtb virulence. The gene expression patterns, growth, and survival of mutants containing transposon insertions in senX3 (senX3::Tn) and regX3 (regX3::Tn) were compared to those of their respective complemented strains and the isogenic wild-type parent strain during axenic growth in nutrient-rich broth, phosphate depletion, nutrient starvation, and in the lungs of BALB/c mice.
  
  
  RESULTS
  regX3 expression was reduced in senX3::Tn during phosphate depletion and nutrient starvation, and expression of the phosphate-specific transport gene pstC2 was reduced similarly in senX3::Tn and regX3::Tn during phosphate depletion. Although senX3 and regX3 were each dispensable for Mtb growth in nutrient-rich broth, disruption of senX3 or regX3 caused a similar growth defect during phosphate depletion. Interestingly, senX3::Tn, in which monocistronic regX3 expression is preserved, showed significantly higher survival relative to regX3::Tn after 7 days of nutrient starvation (p <0.01), and in mouse lungs at Day 31 (p < 0.01), Day 62 (p < 0.01), and Day 124 (p = 0.05) after aerosol infection.
  
  
  CONCLUSIONS
  Our data demonstrate the specificity of the senX3-regX3 2CRS for sensing and responding to low ambient phosphate, but also raise the possibility that RegX3 may function independently of its cognate sensor histidine kinase.
authors.forename: Dalin
authors.initials: D
authors.lastname: Rifat
authors.forename: Deborah A
authors.initials: DA
authors.lastname: Belchis
authors.forename: Petros C
authors.initials: PC
authors.lastname: Karakousis
mesh_terms: D000818:Animals; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D003470:Culture Media / Q000737:chemistry; D004251:DNA Transposable Elements; D004195:Disease Models, Animal; D005260:Female; D020869:Gene Expression Profiling; D015964:Gene Expression Regulation, Bacterial; D005816:Genetic Complementation Test; D008168:Lung / Q000382:microbiology; D008807:Mice, Inbred BALB C; D050296:Microbial Viability; D016254:Mutagenesis, Insertional; D009169:Mycobacterium tuberculosis / Q000235:genetics / Q000254:growth & development / Q000472:pathogenicity / Q000502:physiology*; D010710:Phosphates / Q000378:metabolism*; D010770:Phosphotransferases / Q000235:genetics / Q000378:metabolism*; D014376:Tuberculosis / Q000382:microbiology / Q000473:pathology; D014774:Virulence
chemical_list: D001426:Bacterial Proteins; D003470:Culture Media; D004251:DNA Transposable Elements; D010710:Phosphates; C110608:RegX3 protein, Mycobacterium; D010770:Phosphotransferases; C110607:SenX3 protein, Mycobacterium