AUTOREGULATION OF ESCHERICHIA-COLI PURR REQUIRES 2 CONTROL SITES DOWNSTREAM OF THE PROMOTER

The expression of Escherichia coli purR, which encodes the pur regulon repressor protein, is autoregulated. Autoregulation at the level of transcription requires two operator sites, designated purRo1 and purRo2 (01 and 02). Operator 01 is in the region of DNA between the transcription start site and the site for translation initiation, and 02 is the protein-coding region. The repressor protein binds noncooperatively to 01 with a sixfold-higher affinity than to 02, and saturation of 01 by the represor precedes saturation of 02. Both 01 and 02 function in the two- to threefold autoregulation in vivo, as determined by measurement of β-galactosidase and mRNA from purR-lazZ translational fusions. Of all the genes thus far known to be regulated ty the Pur repressor, only purR employs a two-operator mechanism.