Control of the argECBH cluster in Escherichia coli

The argECBH genes in Escherichia coli are tightly clustered, but argE is not controlled coordinately with argCBH. Furthermore, while nonsense mutations have been isolated in argC and argB which are polar for argH, nonsense and frameshift mutations in argE are found nonpolar for the remaining genes in the cluster. Conditions have been realized for selecting mutations which relieve repression of argC without affecting control of arg genes outside the cluster. These mutations map between argE and argC, are cis-dominant, and cause partial constitutivity for argE as well as for argCBH. These results suggest that the argECBH cluster comprises two operons transcribed divergently from an internal operator-promoter complex.