JB Accepts, published online ahead of print on 23 October 2009

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J. Bacteriol. doi:10.1128/JB.00829-08
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The AraC/XylS Family Activator RhaS Negatively Autoregulates rhaSR Expression by Preventing CRP Activation

Jason R. Wickstrum, Jeff M. Skredenske, Vinitha Balasubramaniam, Kyle Jones, and Susan M. Egan^*

Department of Molecular Biosciences, University of Kansas, Lawrence, Kansas

^* To whom correspondence should be addressed. Email: sme{at}ku.edu.

The Escherichia coli RhaR protein activates expression of the rhaSR operon in the presence of its effector, L-rhamnose. The resulting RhaS protein (plus L-rhamnose) activates expression of the L-rhamnose catabolic and transport operons, rhaBAD and rhaT, respectively. Here, we further investigated our previous finding that rhaS deletion resulted in a three-fold increase in rhaSR promoter activity, suggesting RhaS negative autoregulation of rhaSR. We found that RhaS autoregulation required the CRP binding site at rhaSR, and that RhaS was able to bind to the RhaR binding site at rhaSR. In contrast to the expected repression, we found that in the absence of both RhaR and the CRP binding site at the rhaSR promoter, RhaS activated expression to a level comparable with RhaR activation of the same promoter. However, when the promoter included the RhaR and CRP binding sites, activation by RhaS and CRP was to a much lower level than activation by RhaR and CRP, suggesting that CRP could not fully co-activate with RhaS. Taken together, our results indicate that RhaS negative autoregulation involves RhaS competition with RhaR for binding to the RhaR binding site at rhaSR. Although RhaS and RhaR activate rhaSR transcription to similar levels, CRP cannot effectively co-activate with RhaS. Therefore, once RhaS reaches a relatively high protein concentration, presumably sufficient to saturate the RhaS-activated promoters, there will be a decrease in rhaSR transcription. We propose a model in which differential DNA bending by RhaS and RhaR may be the basis for the difference in CRP co-activation.