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J. Bacteriol. doi:10.1128/JB.01304-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The BlcC (AttM) Lactonase of Agrobacterium tumefaciens Does Not Quench the Quorum-Sensing System That Regulates Ti plasmid Conjugative Transfer

Department of Microbiology, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA

^* To whom correspondence should be addressed. Email: stephenf{at}uiuc.edu.

	Abstract

Conjugative transfer of Agrobacterium plasmids is controlled by a quorum-sensing system consisting of TraR and its acyl-homoserine lactone ligand. The acyl-HSL is essential for TraR-mediated activation of the Ti plasmid Tra genes. Strains A6 and C58 of A. tumefaciens produce a lactonase, BlcC (ex AttM) that can degrade the quormone leading some to conclude that the enzyme quenches the quorum-sensing system. We tested this hypothesis by examining the effects of mutation, induction, or mutational derepression of blcC on accumulation of the acyl-HSL and on conjugative competence of strain C58. Induction of blc resulted in an 8-10 fold decrease in levels of extracellular acyl-HSL, but in only a two-fold decrease in intracellular quormone levels, a measure of the amount of active intracellular TraR. Induction or mutational derepression of blc, as well as a null mutation in blcC had no significant effect on induction of or continued transfer of pTiC58 from donors in any stage of growth including stationary phase. In matings performed in developing tumors, wild-type C58 transferred the Ti plasmid to recipients yielding transconjugants by 14 to 21 days following infection. blcC-null donors yielded transconjugants one week earlier, but by the following week transconjugants were recovered at numbers indistinguishable from wild-type. Donors mutationally derepressed for blcC yielded transconjugants in planta at numbers 10-fold lower than wild-type at weeks two and three, but by week four the two donors showed no difference in recoverable transconjugants. We conclude that BlcC has no biologically-significant effect on Ti plasmid transfer or its regulatory system.