Functional and Expression Analysis of the Metal-InducibledmeRFSystem from Rhizobium leguminosarum bv. viciae

Abstract

ABSTRACTA gene encoding a homolog to the cation diffusion facilitator protein DmeF fromCupriavidus metalliduranshas been identified in the genome ofRhizobium leguminosarumUPM791. TheR. leguminosarum dmeFgene is located downstream of an open reading frame (designateddmeR) encoding a protein homologous to the nickel- and cobalt-responsive transcriptional regulator RcnR fromEscherichia coli. Analysis of gene expression showed that theR. leguminosarum dmeRFgenes are organized as a transcriptional unit whose expression is strongly induced by nickel and cobalt ions, likely by alleviating the repressor activity of DmeR ondmeRFtranscription. AnR. leguminosarum dmeRFmutant strain displayed increased sensitivity to Co(II) and Ni(II), whereas no alterations of its resistance to Cd(II), Cu(II), or Zn(II) were observed. A decrease of symbiotic performance was observed when pea plants inoculated with anR. leguminosarum dmeRFdeletion mutant strain were grown in the presence of high concentrations of nickel and cobalt. The same mutant induced significantly lower activity levels of NiFe hydrogenase in microaerobic cultures. These results indicate that theR. leguminosarumDmeRF system is a metal-responsive efflux mechanism acting as a key element for metal homeostasis inR. leguminosarumunder free-living and symbiotic conditions. The presence of similardmeRFgene clusters in otherRhizobiaceaesuggests that thedmeRFsystem is a conserved mechanism for metal tolerance in legume endosymbiotic bacteria.