PhoP -RCB- PhoQ activates transcription of pmrA -RCB- B .
The tolB effect appears to be specific to the ugd gene because transcription of the PhoP-activated PmrA-dependent pbgP gene and the PhoP-activated PmrA-independent mgtA gene was not affected by the tolB mutation regardless of the presence of functional phoP or pmrA genes ( Fig. 2A ; data not shown ) .
We determined that a ugtL pmrA double mutant was as susceptible to polymyxin B as a phoP mutant ( Fig. 3C ) , which suggests that both types of lipid-A modifications may operate at the same time and argues against the participation of other PhoP-regulated genes in resistance to polymxyin B. Thus , the increased polymyxin B susceptibility reported for mutants defective in the PhoP-activated mig-14 , virK and somA genes may be characteristic of the SL1344 genetic background used in those studies ( Brodsky et al. , 2002 ; Detweiler et al. , 2003 ) because derivatives of the 14028s strain deleted for the mig-14 gene or harbouring a MudJ transposon insertion in the virK gene retained wild-type resistance to both polymxyin B and magainin 2 ( our unpublished results ) .
the regulatory gene pmrA is post-tran-scriptionally activated by the PhoP