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J. Bacteriol. doi:10.1128/JB.00331-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

BigR, a Transcriptional Repressor from Plant-Associated Bacteria, Regulates an Operon Implicated in Biofilm Growth

Center for Molecular and Structural Biology, Brazilian Synchrotron Light Laboratory, Campinas, SP, Brazil, CP6192

^* To whom correspondence should be addressed. Email: celso{at}lnls.br.

	Abstract

Xylella fastidiosa is a plant pathogen that colonizes the xylem vessels causing vascular occlusion due to bacterial biofilm growth. However, little is known about the molecular mechanisms driving biofilm formation in Xylella-plant interactions. Here, we show that BigR (biofilm growth-associated repressor) is a novel helix-turn-helix repressor that controls transcription of an operon implicated in biofilm growth. This operon, which encodes BigR, membrane proteins and an unusual beta-lactamase-like hydrolase (BLH), is restricted to a few plant-associated bacteria, and thus, we sought to understand its regulation and function in X. fastidiosa and Agrobacterium tumefaciens. BigR binds to a palindromic AT-rich element (BigR box) in the Xylella and Agrobacterium blh promoters and strongly represses transcription of the operon in these cells. The BigR box overlaps with two alternative -10 regions identified in the blh promoters and mutations in this box significantly affected transcription, indicating that BigR competes with the RNA polymerase for the same promoter site. Although BigR is similar to members of the ArsR/SmtB family of regulators, our data suggest that it does not act as a metal sensor, as initially predicted. Increased activity of the BigR operon was observed in both Xylella and Agrobacterium biofilms. In addition, A. tumefaciens mutated in the bigR gene showed constitutive expression of operon genes and increased biofilm formation in glass surfaces and tobacco roots, indicating that the operon may play a role in cell adherence or biofilm development.