Transsexuality in the Rhizosphere: Quorum Sensing Reversibly Converts Agrobacterium tumefaciens from Phenotypically Female to Male

Department of Microbiology, 360A Wing Hall, Cornell University, Ithaca, NY 14853, USA.
Journal of bacteriology (Impact Factor: 2.81). 04/2009; 191(10):3375-83. DOI: 10.1128/JB.01608-08
Source: PubMed


Conjugative plasmids generally encode proteins that block the conjugative entry of identical or similar plasmids into the
host cell, a phenomenon known as entry exclusion. Here, we demonstrate that two Ti plasmids of Agrobacterium tumefaciens encode robust entry exclusion functions. Two proteins, TrbJ and TrbK, can each mediate entry exclusion and act synergistically.
The trbJ and trbK genes are included within the trb operon, which is tightly regulated by the quorum-sensing regulator TraR and the cognate acylhomoserine lactone. In the absence
of quorum-sensing signals, these proteins are not significantly expressed, and cells lacking TrbJ and TrbK are efficient Ti
plasmid recipients. In the presence of these signals, these strains block the entry of Ti plasmids and instead become efficient
conjugal donors.

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    • "Since the finding that A. tumefaciens QS controlled Ti plasmid conjugation, a “nagging” question remained to understand the relevance of a system in which donor cells could only monitor the density of other donors that already harbor a Ti-plasmid. Indeed as conjugation cannot happen in a cell already containing a resident Ti plasmid (Cho et al., 2009), the risk of uselessly activating, at the quorum concentration, the horizontal transfer machinery in the absence of sufficiently numerous recipient cells seems elevated. Nonetheless, as evoked previously, the master control of QS by conjugal opines might provide a way to circumvent this difficulty by allowing the conjugation of Ti plasmid only in mature tumors, i.e., in environments where the proportion of recipient cells would have extended. "
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