LuxR-family ‘solos’: bachelor sensors/regulators of signalling molecules

Abstract

N-Acylhomoserine lactone (AHL) quorum-sensing (QS) signalling is the best-understood chemical language in proteobacteria. In the last 15 years a large amount of research in several bacterial species has revealed in detail the genetic, molecular and biochemical mechanisms underlying AHL signalling. These studies have revealed the role played by protein pairs of the AHL synthase belonging to the LuxI family and cognate LuxR-family AHL sensor-regulator. Proteobacteria however commonly possess a QS LuxR-family protein for which there is no obvious cognate LuxI synthase; these proteins are found in bacteria which possess a complete AHL QS system(s) as well as in bacteria that do not. Scientists are beginning to address the roles played by these proteins and it is emerging that they could allow bacteria to respond to endogenous and exogenous signals produced by their neighbours. AHL QS research thus far has mainly focused on a cell-density response involving laboratory monoculture studies. Recent findings on the role played by the unpaired LuxR-family proteins highlight the need to address bacterial behaviour and response to signals in mixed communities. Here we review recent progress with respect to these LuxR proteins, which we propose to call LuxR 'solos' since they act on their own without the need for a cognate signal generator. Initial investigations have revealed that LuxR solos have diverse roles in bacterial interspecies and interkingdom communication.

Exploiting Quorum Sensing To Confuse Bacterial Pathogens

“…Exceptions to this are species that utilize multiple synthase/receptor pairs that generate and respond to distinct AHL molecules, as is the case for P. aeruginosa (Fig. 1B) (7), as well as species that carry so-called "solo" LuxR-type receptors that have no cognate LuxI-type synthase, such as QscR of P. aeruginosa and SidA of Escherichia coli (33). …”

A solo luxI-type gene directs acylhomoserine lactone synthesis and contributes to motility control in the marine sponge symbiont Ruegeria sp. KLH11

“…LuxR solos can regulate gene expression by binding to AHLs produced by other luxI genes encoded elsewhere in the same bacterial genome, such as in the cases of QscR in Pseudomonas aeruginosa (Lequette et al, 2006) and ExpR in Sinorhizobium meliloti (McIntosh et al, 2008), or by binding to AHLs produced by other bacteria, such as in the case of SdiA in Escherichia coli and Salmonella enterica (Ahmer, 2004;Yao et al, 2006). Furthermore, some LuxR-type proteins can regulate gene expression in response to non-AHL signals or independently of ligand binding (Subramoni & Venturi, 2009). Although progress has been made in understanding these luxR solos, there is little information about the less common scenario in which functional luxI genes are not linked to luxR-type genes, which we designate as luxI solos.…”

“…However, in many different bacteria, luxR-type genes without a linked luxI-type gene have been discovered and these luxR-type genes are termed luxR solos. These solos occur both in bacteria that have complete LuxRLuxI-type QS systems and bacteria that do not (Subramoni & Venturi, 2009). LuxR solos can regulate gene expression by binding to AHLs produced by other luxI genes encoded elsewhere in the same bacterial genome, such as in the cases of QscR in Pseudomonas aeruginosa (Lequette et al, 2006) and ExpR in Sinorhizobium meliloti (McIntosh et al, 2008), or by binding to AHLs produced by other bacteria, such as in the case of SdiA in Escherichia coli and Salmonella enterica (Ahmer, 2004;Yao et al, 2006).…”

Dialkylresorcinols as bacterial signaling molecules

“…The prototypical quorum sensing system of Gram-negative bacteria consists of a LuxI-like autoinducer synthase that produces acyl-homoserine lactones (AHLs) as signals and a LuxR-type receptor that detects the AHLs to control expression of specific genes (1). Usually, luxI/luxR pairs are genetically clustered; however, there are examples in which the luxI/luxR functional pairs are distantly located on the bacterial chromosome or on plasmids (2). The AHLs are synthesized by LuxI and are sensed by the cognate LuxR-type receptor when exceeding a threshold concentration.…”

“…Those LuxR homologs are designated as LuxR orphans (4) or LuxR solos (2). LuxR solos have been found in AHL-producing as well as in non-AHL-producing bacteria.…”