SwrA regulates assembly of Bacillus subtilis DegU via its interaction with N-terminal domain of DegU.

The Bacillus subtilis response regulator DegU controls many physiological events including swarming motility and exoprotease production. Swarming motility is a multicellular movement of hyper-flagellated cells on a surface. The swarming motility regulator SwrA and DegU cooperatively drive transcription of fla/che encoding flagella components, chemotaxis constituents and motility-specific sigma factor, which is regarded as the primary event in the development of motility. We have identified ycdA involved in swarming motility, encoding a putative lipoprotein. We showed that the ycdA gene is positively regulated by DegU and SwrA. Mutational analysis of ycdA-lacZ revealed that SwrA changes the use of cis-acting sites for DegU. This suggested that SwrA operates the DegU-regulation mode through changes in the DegU assembly state. DegU binding to the ycdA-promoter region carrying an unusual arrangement of DegU-recognition sequences with low affinity was found to be stimulated by SwrA in electrophoretic mobility shift assay and DNase I footprinting. Yeast two- and three-hybrid analyses revealed that the N-terminal domain of DegU interacts with whole DegU, which is facilitated by SwrA. Together, these results demonstrate that SwrA can stabilize the binding of DegU to the ycdA promoter with low affinity. Thus, SwrA is a novel type of bacterial transcription factor in this regard.