When system is activated independently of PmrD , such as by exposure to Fe3 , the PmrA protein represses pmrD transcription .
When the Salmonella PmrA PmrB is activated independently of PmrD , such as by exposure to Fe3 , the PmrA protein represses pmrD transcription .
We reasoned that if the PmrA-mediated repression of the Salmonella pmrD gene is designed to prevent the potentially detrimental production of PmrD protein , this negative feedback loop would be absent from E. coli because its PmrD protein is unable to activate the PmrA protein .
For example , the presence of a PmrA-binding site in the Salmonella pmrD promoter enables the PmrA protein to repress pmrD transcription
The PhoP -- PmrA loop is closed by repression of pmrD expression by PmrA .
The PhoP -- PmrD is closed by repression of pmrD expression by PmrA .
This loop can be deactivated by the transcriptional repression of pmrD by PmrA .