title: Targeting RNA polymerase primary σ70 as a therapeutic strategy against methicillin-resistant Staphylococcus aureus by antisense peptide nucleic acid.
abstract: BACKGROUND
  Methicillin-resistant Staphylococcus aureus (MRSA) causes threatening infection-related mortality worldwide. Currently, spread of multi-drug resistance (MDR) MRSA limits therapeutic options and requires new approaches to "druggable" target discovery, as well as development of novel MRSA-active antibiotics. RNA polymerase primary σ⁷⁰ (encoded by gene rpoD) is a highly conserved prokaryotic factor essential for transcription initiation in exponentially growing cells of diverse S. aureus, implying potential for antisense inhibition.
  
  
  RESULTS
  By synthesizing a serial of cell penetrating peptide conjugated peptide nucleic acids (PPNAs) based on software predicted parameters and further design optimization, we identified a target sequence (234 to 243 nt) within rpoD mRNA conserved region 3.0 being more sensitive to antisense inhibition. A (KFF)₃K peptide conjugated 10-mer complementary PNA (PPNA2332) was developed for potent micromolar-range growth inhibitory effects against four pathogenic S. aureus strains with different resistance phenotypes, including clinical vancomycin-intermediate resistance S. aureus and MDR-MRSA isolates. PPNA2332 showed bacteriocidal antisense effect at 3.2 fold of MIC value against MRSA/VISA Mu50, and its sequence specificity was demonstrated in that PPNA with scrambled PNA sequence (Scr PPNA2332) exhibited no growth inhibitory effect at higher concentrations. Also, PPNA2332 specifically interferes with rpoD mRNA, inhibiting translation of its protein product σ⁷⁰ in a concentration-dependent manner. Full decay of mRNA and suppressed expression of σ⁷⁰ were observed for 40 µM or 12.5 µM PPNA2332 treatment, respectively, but not for 40 µM Scr PPNA2332 treatment in pure culture of MRSA/VISA Mu50 strain. PPNA2332 (≥1 µM) essentially cleared lethal MRSA/VISA Mu50 infection in epithelial cell cultures, and eliminated viable bacterial cells in a time- and concentration- dependent manner, without showing any apparent toxicity at 10 µM.
  
  
  CONCLUSIONS
  The present result suggested that RNAP primary σ⁷⁰ is a very promising candidate target for developing novel antisense antibiotic to treat severe MRSA infections.
authors.affiliation: Department of Pharmacology, School of Pharmacy, Fourth Military Medical University, Xi'an, Shaanxi, China.
authors.forename: Hui
authors.initials: H
authors.lastname: Bai
authors.forename: Guojun
authors.initials: G
authors.lastname: Sang
authors.forename: Yu
authors.initials: Y
authors.lastname: You
authors.forename: Xiaoyan
authors.initials: X
authors.lastname: Xue
authors.forename: Ying
authors.initials: Y
authors.lastname: Zhou
authors.forename: Zheng
authors.initials: Z
authors.lastname: Hou
authors.forename: Jingru
authors.initials: J
authors.lastname: Meng
authors.forename: Xiaoxing
authors.initials: X
authors.lastname: Luo
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology / Q000627:therapeutic use; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D001483:Base Sequence; D002478:Cells, Cultured; D012321:DNA-Directed RNA Polymerases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D004847:Epithelial Cells / Q000187:drug effects / Q000382:microbiology / Q000473:pathology; D015964:Gene Expression Regulation, Bacterial / Q000187:drug effects; D006801:Humans; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects* / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development; D008826:Microbial Sensitivity Tests; D050296:Microbial Viability / Q000187:drug effects; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D020135:Peptide Nucleic Acids / Q000494:pharmacology* / Q000627:therapeutic use*; D020011:Protective Agents / Q000494:pharmacology; D016372:RNA, Antisense / Q000494:pharmacology / Q000627:therapeutic use*; D012333:RNA, Messenger / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D012808:Sigma Factor / Q000037:antagonists & inhibitors* / Q000378:metabolism; D013203:Staphylococcal Infections / Q000188:drug therapy* / Q000382:microbiology
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D020135:Peptide Nucleic Acids; D020011:Protective Agents; D016372:RNA, Antisense; D012333:RNA, Messenger; D012808:Sigma Factor; C081775:RNA polymerase sigma 70; D012321:DNA-Directed RNA Polymerases