Quorum Sensing but Not Autoinduction of Ti Plasmid Conjugal Transfer Requires Control by the Opine Regulon and the Antiactivator TraM

doi:10.1128/JB.182.4.1080-1088.2000

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Journal of Bacteriology, February 2000, p. 1080-1088, Vol. 182, No. 4
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Quorum Sensing but Not Autoinduction of Ti Plasmid Conjugal Transfer Requires Control by the Opine Regulon and the Antiactivator TraM

Kevin R. Piper¹ and Stephen K. Farrand¹^,²^,^*

Departments of Microbiology¹ and Crop Sciences,² University of Illinois at Urbana-Champaign, Urbana, Illinois 61801

Received 13 September 1999/Accepted 19 November 1999

Conjugal transfer of the Ti plasmids from Agrobacterium tumefaciens is controlled by autoinduction via the transcriptionalactivator TraR and the acyl-homoserine lactone ligand, Agrobacteriumautoinducer (AAI). This control process is itself regulated byopines, which are small carbon compounds produced by the crowngall tumors that are induced by the bacteria. Opines control autoinductionby regulating the expression of traR. Transfer of pTiC58 fromdonors grown with agrocinopines A and B, the conjugal opines forthis Ti plasmid, was detected only after the donors had reacheda population level of 10⁷ cells per cm². Donors incubated with the opines and AAI transferred their Tiplasmids at population levels about 10-fold lower than those incubatedwith opines only. Transcription of the tra regulon, as assessedby monitoring a traA::lacZ reporter, showed a similar dependenceon the density of the donor population. However, even in culturesat low population densities that were induced with opines andAAI, there was a temporal lag of between 15 and 20 h in the developmentof conjugal competence. Moreover, even after this latent period,maximal transfer frequencies required several hours to develop.This lag period was independent of the population density of thedonors but could be reduced somewhat by addition of exogenousAAI. Quorum-dependent development of conjugal competence requiredcontrol by the opine regulon; donors harboring a mutant of pTiC58deleted for the master opine responsive repressor accR transferredthe Ti plasmid at maximum frequencies at very low population densities.Similarly, an otherwise wild-type derivative of pTiC58 lackingtraM, which codes for an antiactivator that inhibits TraR activity,transferred at high frequency in a population-independent mannerin the absence of the conjugal opines. Thus, while quorum sensingis dependent upon autoinduction, the two phenomena are not synonymous.We conclude that conjugal transfer of pTiC58 is regulated in aquorum-dependent fashion but that supercontrol of the TraR-AAIsystem by opines and by TraM results in a complex control processthat requires not only the accumulation of AAI but also the expressionof TraR and the synthesis of this protein at levels that overcomethe inhibitory activity ofTraM.

^* Corresponding author. Mailing address: Department of Crop Sciences, University of Illinois at Urbana-Champaign, 240 EdwardR. Madigan Laboratory, 1201 West Gregory Dr., Urbana, IL 61801.Phone: (217) 333-1524. Fax: (217) 244-7830. E-mail: stephenf{at}uiuc.edu.

Journal of Bacteriology, February 2000, p. 1080-1088, Vol. 182, No. 4
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

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