PQS and pyochelin in Pseudomonas aeruginosa share inner membrane transporters to mediate iron uptake

Abstract

Bacteria uptake different forms of iron through various channels to meet their needs. Our previous studies have shown that TseF, a type VI secretion system effector for Fe uptake, facilitates the delivery of outer membrane vesicle (OMV)-associated PQS-Fe3+ to bacterial cells by involving the Fe(III) pyochelin receptor FptA and the porin OprF. However, the form in which the PQS-Fe3+ complex enters the periplasm and how it is taken up into the cytoplasm remain unclear. Here, we first demonstrate that the PQS-Fe3+ complex enters the cell directly through FptA or OprF. Next, we find that inner membrane transporters such as FptX, PchHI, and FepBCDG are not only necessary for P. aeruginosa to uptake PQS-Fe3+ and PCH-Fe3+, but also necessary for the virulence of P. aeruginosa toward Galleria mellonella larvae. Furthermore, we suggest that the function of PQS-Fe3+ (but not PQS)-mediated quorum-sensing regulation is dependent on FptX, PchHI, and FepBCDG. Additionally, the findings indicate that, unlike FptX, both FepBCDG and PchHI play no role in the autoregulatory loop involving PchR, but further deletion of fepBCDG and pchHI can reverse the inactive PchR phenotype caused by fptX deletion and reactivate the expression of the PCH pathway genes under iron-limited conditions. Finally, this work detected the interaction between FptX, PchHI, and FepBCDG, indicating that a larger complex could be formed to mediate uptake of PQS-Fe3+ and PCH-Fe3+. These results pave the way for a better understanding of the PQS and PCH iron uptake pathway, and provide future directions to tackle P. aeruginosa infections. IMPORTANCEIron is a key factor for P. aeruginosa to break through the hosts defense system and successfully infect. To acquire the necessary iron from the host, P. aeruginosa has evolved a number of strategies, the most common being the synthesis, secretion, and uptake of siderophores such as pyoverdine, pyochelin, and the quorum-sensing signaling molecule PQS. However, despite intensive studies of the siderophore uptake pathways of P. aeruginosa, our understanding of how siderophores transport iron across the inner membrane into the cytoplasm is still far from complete. Here, we reveal that PQS and pyochelin in P. aeruginosa share inner membrane transporters such as FptX, PchHI and FepBCDG to mediate iron uptake. Meanwhile, PQS and pyochelin-mediated signaling operates to a large extent via these inner membrane transporters. Our study revealed an interesting phenomenon of shared uptake pathways between PQS and pyochelin, which will lead us to reexamine the role of these two molecules in the iron uptake and virulence of P. aeruginosa.