Mutations upregulating the flhDC operon of Escherichia coli K-12.

Bacterial motility is governed by the flhDC master operon that is under the control of factors like OmpR, LrhA, HdfR, and H-NS. Previously, derivatives of the wild-type MG1655 strain of E. coli K-12 with enhanced motility were found to contain insertion sequences (ISs) in the regulatory region of the flhDC operon. Here, we report that not only integrations of IS insertion sequences into the regulatory region of the flhDC operon, but also a missense mutation in the lrhA gene enhances motility by relieving transcriptional repression of the flhDC operon. Two novel IS insertions were found upstream of flhDC. So far, the relationships between the trans- acting factors and the cis-acting regulatory sequences associated with the flhDC operon have not been clearly established. In this study, it was found that effects of the cis- and trans-acting mutations were acting in parallel, suggesting their apparently independent regulation of flagellar expression.