title: Genetic characterization of compensatory evolution in strains carrying rpoB Ser531Leu, the rifampicin resistance mutation most frequently found in clinical isolates.
abstract: OBJECTIVE
  The evolution of rifampicin resistance in Mycobacterium tuberculosis is a major threat to effective tuberculosis therapy. Much is known about the initial emergence of rifampicin resistance, but the further evolution of these resistant strains has only lately been subject to investigation. Although resistance can be caused by many different mutations in rpoB, among clinical M. tuberculosis isolates the mutation rpoB S531L is overwhelmingly the most frequently found. Clinical isolates with rpoB S531L frequently carry additional mutations in genes for RNA polymerase subunits, and it has been speculated that these are fitness-compensatory mutations, ameliorating the fitness cost of the primary resistance mutation. We tested this hypothesis using Salmonella as a model organism.
  
  
  METHODS
  We created the rpoB S531L mutation in Salmonella and then evolved independent lineages with selection for mutants with increased relative fitness. Relative fitness associated with putative compensatory mutations was measured after genetic reconstruction in isogenic strains.
  
  
  RESULTS
  Compensatory mutations were identified in genes coding for different subunits of RNA polymerase: rpoA, rpoB and rpoC. Genetic reconstructions demonstrated that each of these secondary mutations reduced the fitness cost of the rpoB S531L resistance mutation.
  
  
  CONCLUSIONS
  The compensatory mutations identified in Salmonella cluster in similar locations to the additional mutations found in M. tuberculosis isolates. These new data strongly support the idea that many of the previously identified rpoA, rpoB and rpoC mutations in rifampicin-resistant M. tuberculosis (rpoB S531L) are indeed fitness-compensatory mutations.
authors.affiliation: Department of Medical Biochemistry and Microbiology, Box 582 Biomedical Center, Uppsala University, Uppsala, Sweden.
authors.forename: Gerrit
authors.initials: G
authors.lastname: Brandis
authors.forename: Diarmaid
authors.initials: D
authors.lastname: Hughes
mesh_terms: D000995:Antitubercular Agents / Q000494:pharmacology*; D012321:DNA-Directed RNA Polymerases / Q000235:genetics*; D024881:Drug Resistance, Bacterial; D020125:Mutation, Missense; D009169:Mycobacterium tuberculosis / Q000187:drug effects* / Q000235:genetics* / Q000502:physiology; D012293:Rifampin / Q000494:pharmacology*; D012475:Salmonella / Q000235:genetics / Q000502:physiology
chemical_list: D000995:Antitubercular Agents; D012321:DNA-Directed RNA Polymerases; C105078:RNA polymerase beta subunit; D012293:Rifampin
keywords: Mycobacterium tuberculosis; S531L; antibiotic resistance