Regulation by PAS_4/LuxR-like receptors 
In P. luminescens, the amount of luxR-like genes is overrepresented with 39 copies in the genome.
35 of these potential LuxR-like receptors exhibit PAS_4 signal binding domains instead of an AHL-binding domain, and two have a signalling domain with a yet unidentified motif (Fig. 3).
Most of the genes are located in two large gene clusters (plu0918-0925 and plu2001-2019).
Interestingly, eleven of those LuxR-like receptors are present in Y. enterocolitica of which five are located in a cluster (ye0035-0039).
Nine of these have a so-called PAS_4 signal binding domains of yet unknown function.
It is interesting to note that there is only one bacterium else, the insect colonizing S. glossinidius, whose genome also carries a series of unclustered genes coding for PAS_4/LuxR-like receptors, indicating that this kind of receptors plays a role during insect infection.
PAS-domains have been suspected to act as insect juvenile hormone (JH) receptors in the fruit fly Drosophila melanogaster [43,44].
The methoprene-tolerant gene (met, also called Resistance to Juvenile Hormone Rst1JH) of D. melanogaster encodes a helix-loop-helix transcriptional regulator combined with a PAS_3 domain [45].
Met has been shown to bind JH at physiological concentrations and is therefore suspected to act as a JH receptor [46,47].
Therefore, the potential PAS_4/LuxR-like receptors of P. luminescens, Y. enterocolitica, and S. glossinidius might sense JH or other eukaryotic hormones of the insect to adapt their gene expression to the insect host.
The high number of 35 highly homologous receptor proteins in P. luminescens might be the reason for the wide insect host spectrum this pathogen is capable to infect.
Although Y. enterocolitica protein extracts confer toxicity against M. sexta larvae [7], its host spectrum still remains to be defined.
The difference in the number of the uncommon LuxR-like receptors (35 in P. luminescens, nine in Y. enterocolitica) gives rise to speculations that the insect host spectrum is constricted for Y. enterocolitica compared with P. luminescens.
This hypothesis is underlined by the fact that not more than five PAS_4/LuxR-like receptors are present in S. glossinidius for which only one insect host has been reported.
