Regulatory role of MlrA in transcription activation of csgD, the master regulator of biofilm formation in Escherichia coli

The transcription factor CsgD plays a key role in the control of biofilm formation in Escherichia coli by controlling the production of curli fimbriae and other biofilm components. In concert with its regulatory role, the promoter for the csgD operon is under the control of more than 10 regulatory factors, each monitoring a different condition or factor in stressful environments. Previously, we classified three factors (OmpR, RstA and IHF) as activators and two factors (CpxR and H-NS) as repressors, and found novel modes of their interplay. Here we describe an as yet uncharacterized regulator, MlrA, that has been suggested to participate in control of curli formation. Based on both in vivo and in vitro analyses, we identified MlrA as a positive factor of the csgD promoter by directly binding to its upstream region (-113 to -146) with a palindromic sequence of AAAATTGTACA(12N)TGTACAATTTT between the binding sites of two activators, IHF and OmpR. The possible interplay between three activators was analysed in detail.