terminus and a helix-turn-helix DNA-binding domain at their
C-terminus. LuxR solos have so far been implicated in intraspecies,
interspecies, and interkingdom communication and are likely to
play a major role in cell-cell communication in bacteria. Some of
them have been shown to bind AHLs and expand the regulatory
repertoire of a classical quorum-sensing system by responding to
endogenous AHLs [8]. Alternatively they can also regulate genes
by “eavesdropping” on exogenous AHLs produced by neighboring
bacteria. QscR ofPseudomonas aeruginosais a very-well-studied
example of a LuxR solo responding to endogenously produced
AHLs [25] and SdiA ofE. coliandSalmonella entericais a well-
studied LuxR solo which binds exogenous AHLs produced by
neighbors [26]. Recently a group of non-AHL-binding LuxR
solos has been identified that bind yet-unknown plant-derived
molecules, and these have been studied in several plant-associated
bacteria [27]. Finally, some LuxR solos have been reported to
regulate target genes in a ligand-independent manner [8, 28].
This chapter intends to highlight three inexpensive and simple
ways to begin to investigate this family of proteins that are currently
under-studied and are likely to play important roles in different
types of cell-cell bacterial communication. These approaches can
then lead to other methodologies allowing further characterization
of LuxR solos (Fig.1). We first describe criteria that can be used for
in silico discrimination of LuxR solos from other QS LuxRs and to
classify them into different LuxR solo types based on certain
sequence features that reflect ligand specificities. In the next sectionFig. 1The three simple methodologies allow a classification of the LuxR solo, information on the ligand, and
identification of potential targets. This could then lead to further studies involving analytical chemistry in order
to precisely determine the structure of the ligand, to study phenotypes and rolein vivo. More modern
approaches like RNAseq and ChIPseq which are expensive and which require extensive validation studies
can also be used for determination of LuxR solo targets
Solo/Orphan QS Receptors 147