Iron uptake 
Bacteria use two different strategies to acquire sufficient amounts of iron, namely the expression and secretion of high-affinity iron-binding compounds called siderophores, and the production of receptors for iron carriers such as heme. Genes involved in the biosynthesis, transport and regulation of the siderophore yersiniabactin are clustered in the high pathogenicity island of Y. enterocolitica [103] and have counterparts in P. luminescens (plu2316-2324). Remarkably, yersiniabactin is absent in all Y. enterocolitica strains beside biovar 1B. Present in both bacterial organisms compared here are also genes encoding a hemine uptake system (ye0323-0332/plu2631-2636), the YfeABCD transporter system of chelated iron, the ferrous (Fe2+) iron transporter proteins FeoAB, the AfuABC/SfuABC ferric (Fe3+) transporter, the enterobactin and its transporter (FepBDCG), the FecABCDE ABC transporter system, and several putative hemin/siderophore/iron uptake proteins (YE1459-1461/Plu2850-2852), YE3190/Plu2853, and YE0555/Plu3738). The proteins encoded by the P. luminescens fecIRABCDE operon are similar to the components of the E. coli Fe3+-dicitrate transport system. Homologues are present in the genome of Y. enterocolitica, but scattered over the chromosome. In addition, Y. enterocolitica produces two heme-protein acquisition sytems (YE0123-126, YE2180-2182), a second SfuABC system, the ferrichrome binding and transport proteins (YE0730-0732), a putative siderophore (YE0704), and a hemin storage system (YE2481-2484). None of these iron acquisition systems is present in P. luminescens which in contrast produces the siderophore photobactin [104]. Furthermore, P. luminescens encodes two putative heme-binding hemopexin-like proteins, the photopexins PpxA (Plu4242) and PpxB (Plu4243), which are the first hemopexins found in bacteria. It is suggested that the photopexins may be used by P. luminescens to scavenge iron containing compounds from insects [105]. Interestingly, three gene loci involved in iron acquisition, namely the genes encoding the hemin storage system, the yersiniabactin and the enterobactin transporter FepG, have been demonstrated to be up-regulated upon temperature decrease in Y. pestis or Y. ruckeri, respectively (Table 1). This large set of iron, hemin, heme and siderophore transporters underlines the importance of iron availability for the life cycles of P. luminescens and Y. enterocolitica. It also indicates that iron acquisition is a prerequisite for the infection process of pathogenic bacteria not only in mammalian, but also in invertebrate hosts, and underlines the suggestion that genetic determinants of invertebrate pathogens such as P. luminescens include the progenitors of virulence factors against vertebrates [79,106].
