Quorum-dependent transfer of the opine-catabolic plasmid pAoF64/95 is regulated by a novel mechanism involving inhibition of the TraR antiactivator TraM

Molecular Mechanisms and Applications of N-Acyl Homoserine Lactone-Mediated Quorum Sensing in Bacteria

Microbial biodiversity includes biotic and abiotic components that support all life forms by adapting to environmental conditions. Climate change, pollution, human activity, and natural calamities affect microbial biodiversity. Microbes have diverse growth conditions, physiology, and metabolism. Bacteria use signaling systems such as quorum sensing (QS) to regulate cellular interactions via small chemical signaling molecules which also help with adaptation under undesirable survival conditions. Proteobacteria use acyl-homoserine lactone (AHL) molecules as autoinducers to sense population density and modulate gene expression. The LuxI-type enzymes synthesize AHL molecules, while the LuxR-type proteins (AHL transcriptional regulators) bind to AHLs to regulate QS-dependent gene expression. Diverse AHLs have been identified, and the diversity extends to AHL synthases and AHL receptors. This review comprehensively explains the molecular diversity of AHL signaling components of Pseudomonas aeruginosa, Chromobacterium violaceum, Agrobacterium tumefaciens, and Escherichia coli. The regulatory mechanism of AHL signaling is also highlighted in this review, which adds to the current understanding of AHL signaling in Gram-negative bacteria. We summarize molecular diversity among well-studied QS systems and recent advances in the role of QS proteins in bacterial cellular signaling pathways. This review describes AHL-dependent QS details in bacteria that can be employed to understand their features, improve environmental adaptation, and develop broad biomolecule-based biotechnological applications.

Conjugative transfer between Rhizobium etli endosymbionts inside the root nodule

Since the discovery that biological nitrogen fixation ensues in nodules resulting from the interaction of rhizobia with legumes, nodules were thought to be exclusive for hosting nitrogen-fixing and plant growth promoting bacteria. In this work, we uncover a novel function of nodules, as a niche permissive to acquisition of plasmids via conjugative transfer. We used Rhizobium etli CFN42, which nodulates Phaseolus vulgaris. The genome of R. etli CFN42 contains a chromosome and six plasmids. pRet42a is a conjugative plasmid regulated by Quorum-Sensing (QS), and pRet42d is the symbiotic plasmid. Here, using confocal microscopy and flow cytometry, we show that pRet42a transfers on the root's surface, and unexpectedly, inside the nodules. Conjugation still took place inside nodules, even when it was restricted on the plant surface by placing the QS traI regulator under the promoter of the nitrogenase gene, which is only expressed inside the nodules, or by inhibiting the QS transcriptional induction of transfer genes with a traM antiactivator on an unstable vector maintained on the plant surface and lost inside the nodules. These results conclusively confirm the occurrence of conjugation in these structures, defining them as a protected environment for bacterial diversification.

Quorum-dependent transfer of the opine-catabolic plasmid pAoF64/95 is regulated by a novel mechanism involving inhibition of the TraR antiactivator TraM

We previously described a plasmid of Agrobacterium spp., pAoF64/95, in which the quorum-sensing system that controls conjugative transfer is induced by the opine mannopine. We also showed that the quorum-sensing regulators TraR, TraM, and TraI function similarly to their counterparts in other repABC plasmids. However, traR, unlike its counterpart on Ti plasmids, is monocistronic and not located in an operon that is inducible by the conjugative opine. Here, we report that both traR and traM are expressed constitutively and not regulated by growth with mannopine. We report two additional regulatory genes, mrtR and tmsP, that are involved in a novel mechanism of control of TraR activity. Both genes are located in the distantly linked region of pAoF64/95 encoding mannopine utilization. MrtR, in the absence of mannopine, represses the four-gene mocC operon as well as tmsP, which is the distal gene of the eight-gene motA operon. As judged by a bacterial two-hybrid analysis, TmsP, which shows amino acid sequence relatedness with the TraM-binding domain of TraR, interacts with the antiactivator. We propose a model in which mannopine, acting through the repressor MrtR, induces expression of TmsP which then titrates the levels of TraM thereby freeing TraR to activate the tra regulon.