to be encoded by the same genome, it is not appropriate to
refer to these LuxR proteins as LuxR solos. Instead, they are
non-AHL-binding LuxR proteins that sense endogenous
ligands and together constitute the non-AHL QS systems.
However, they might differ from typical AHL QS system in
lacking the feedback effect of LuxR homolog on the signal
synthase, for example the PluR/PpyS system [47].- LuxR solos identified in this manner might be occurring alone
or together with one or more complete QS system(s). - Here TraR is used as an example of a canonical AHL-binding
QS LuxR protein. Other well-studied QS LuxR proteins that
may be used for the purpose of identification of invariant amino
acid residues in the LuxR solo include well-characterized LuxR
homologs such as LasR ofP. aeruginosaand LuxR ofVibrio
fischeri. - The two amino acid substitutions for PAB LuxR solos are
methionine for tryptophan at position 57 and tryptophan for
tyrosine at position 61 in case of OryR [39]. Depending on the
type of LuxR solo, different well-studied representatives of
each type may be used for alignment. For example, for PAB
LuxRs, XccR ofXanthomonas campestris,OryRofXanthomo-
nas oryzae, PsoR ofPseudomonas protegens, or NesR ofSinor-
hizobium melilotimay be aligned with the protein of interest.
For non-AHL LuxR solo, PluR ofP. luminiscensand PauR ofP.
asymbioticamay be used for protein sequence alignment. - It cannot be excluded that some LuxR solo proteins when
purified as unbound to AHLs are soluble and stable [48]. - Since there are many structurally different AHLs available, a
cocktail can be used always ensuring that each AHL is used at a
concentration of 20μM. Subsequently they can be indepen-
dently used in order to establish which one(s) are causing
solubilization of the protein. - It is possible that some plant extracts could be toxic for bacte-
rial growth at these concentrations: in this case reduce the
amount of macerated material accordingly. - The selective conditions must prevent growth of the donor and
recipient strains, but allow growth of transconjugants in which
the transposon has transposed from the suicide vector into the
chromosome of the recipient. Most commonly, this is done
with either nutrient source limitations or antibiotics. In the
case illustrated here, the donor is killed by nalidixic acid while
the recipient is killed by kanamycin. Only transposition events
in the recipient can lead to resistance to both antibiotics. - If no colonies are obtained there may be a problem with the
suicide vector. Plasmids encoding transposons are not stable
156 Vittorio Venturi et al.