Universal stress proteins 
Universal stress proteins (Usp) are small soluble proteins found in bacteria, archaea and plants.
The production of these proteins is induced upon global stress conditions such as nutrient starvation, heat stress, osmotic stress, oxidative stress, or the presence of toxic compounds.
The protein family is divided into the UspA subfamily and the UspFG subfamily.
The functional mechanism of these Usp proteins is not known [48].
Because P. luminescens and Y. enterocolitica are exposed to those stresses upon infecting and colonizing the insect host, we compared their set of Usp proteins (Table 2).
Both genomes share an UspA-like (Plu0121 and YE4050) and an UspE-like (Plu2178 and YE2076) homologue.
In E. coli, the sequence motif of Usp proteins is not highly conserved: UspA and UspC show a sequence identity of 37% and a homology of 57%, for example.
In contrast, the UspA and the UspE homologues of P. luminescens and Y. enterocolitica are nearly similar, indicating that an identical stress response is regulated by these proteins.
Homologues of these proteins are also present in P. aeruginosa, namely PA4352 and PA3309, a tandem-type Usp protein and a UspA-like protein, respectively.
They are essential for survival under anaerobic growth and therefore biofilm formation, a situation cells are exposed to when colonizing the cystic fibrosis lung in hosts [49,50].
The Usp homologues of P. luminescens and Y. enterocolitica might also be important during infection of the insect host.
In P. luminescens, expression of UspA has been shown to be under control of the AstS/AstR TCS, which is important for the correct timing of phase variant switching [28].
It is discussed that the AstS/AstR-system prevents or delays phenotypic variation by protecting the cell from stress [18].
Because Y. enterocolitica produces the corresponding TCS BvgS/BvgR, but is not known to switch to another phenotypic variant, the possible role of UspA in global regulation still remains to be elucidated.
Phenotypic variation and thus the switch between mutualism and pathogenicity in P. luminescens is proposed to be regulated by a Ner-like and a HexA-like regulator that repress primary variant specific genes in the stage of the secondary variant [17].
Therefore, UspA might have a global importance in P. luminescens notifying stress and transmitting signals for HexA [18].
In Y. enterocolitica, the transcriptional repressor RovM (YE1343) is similar to HexA of P. luminescens (61% identity and 75% homology), and has only recently been shown to control cell invasion, virulence and motility in Y. pseudotuberculosis, Y. pestis and Y. enterocolitica [51-53].
This fact suggests a similar UspA-dependent regulatory mechanism used by the two bacteria compared here.
P. luminescens, but not Y. enterocolitica, produces two members of the UspFG family, the UspG homologues Plu2030 and Plu2032 (Tab. 2), indicating a global stress response induced by those Usp proteins that is different in both organisms.
It is known that UspG of E. coli interacts with the chaperonin GroEL [54], which promotes the correct folding of many cytosolic proteins [55].
A GroEL homologue is present in P. luminescens (Plu4134) which the P. luminescens UspG homologues might interact with.
In contrast to P. luminescens, Y. enterocolitica encodes another member of the UspA subfamily, the UspC homologue YE2583 (Tab. 2), which is not present in P. luminescens.
Therefore, an UspC mediated stress response is not assumed to play a major role in insect pathogenicity.
Summarizing, the set of the shared and different Usp proteins reveals a partially similar and a partially different (fine)-regulation of the global stress response modules in P. luminescens and Y. enterocolitica.
This pattern corresponds to the overlapping life cycles of both pathogens (Fig. 1).
The UspA and the UspE homologues are predicted here to be relevant for insect infection, whereas UspC is assumed be more important for Y. enterocolitica in other environments/hosts.
The two UspG homologues might constitute a set of Usp proteins that play a specific role in P. luminescens infection or in symbiosis with the nematode host.
