Legionella pneumophila LbtU Acts as a Novel, TonB-Independent Receptor for the Legiobactin Siderophore

Abstract

Gram-negative Legionella pneumophila produces a siderophore (legiobactin) that promotes lung infection. We previously determined that lbtA and lbtB are required for the synthesis and secretion of legiobactin. DNA sequence and reverse transcription-PCR (RT-PCR) analyses now reveal the presence of an iron-repressed gene (lbtU) directly upstream of the lbtAB-containing operon. In silico analysis predicted that LbtU is an outer membrane protein consisting of a 16-stranded transmembrane ␤-barrel, multiple extracellular domains, and short periplasmic tails. Immunoblot analysis of cell fractions confirmed an outer membrane location for LbtU. Although replicating normally in standard media, lbtU mutants, like lbtA mutants, were impaired for growth on iron-depleted agar media. While producing typical levels of legiobactin, lbtU mutants were unable to use supplied legiobactin to stimulate growth on iron-depleted media and displayed an inability to take up iron. Complemented lbtU mutants behaved as the wild type did. The lbtU mutants were also impaired for infection in a legiobactin-dependent manner. Together, these data indicate that LbtU is involved in the uptake of legiobactin and, based upon its location, is most likely the Legionella siderophore receptor. The sequence and predicted two-dimensional (2D) and 3D structures of LbtU were distinct from those of all known siderophore receptors, which generally contain a 22-stranded ␤-barrel and an extended N terminus that binds TonB in order to transduce energy from the inner membrane. This observation coupled with the fact that L. pneumophila does not encode TonB suggests that LbtU is a new type of receptor that participates in a form of iron uptake that is mechanistically distinct from the existing paradigm.

The Yfe and Feo Transporters Are Involved in Microaerobic Growth and Virulence of Yersinia pestis in Bubonic Plague

“…For example, Pseudomonas siderophores have been detected in the sputum of infected cystic fibrosis patients. In addition, mutations in the siderophore receptor LbtU in Legionella and the Ybt siderophore system of Y. pestis cause a significant loss of virulence during pneumonic infections (17,28,51). Thus, it would appear that in Y. pestis, ferric iron transport via the Ybt system plays a more important role than ferrous iron transport in a pneumonic mouse model of plague.…”

Paralogous Outer Membrane Proteins Mediate Uptake of Different Forms of Iron and Synergistically Govern Virulence in Francisella tularensis tularensis

“…How the transport is facilitated by FslE across the outer membrane in F. tularensis in the absence of a TonB complex remains to be determined. An analogous TonB-independent siderophore uptake system has been recently identified in Legionella pneumophila (42), suggesting that this paradigm may extend to other bacteria lacking TonB.…”

The novel Legionella pneumophila type II secretion substrate NttC contributes to infection of amoebae Hartmannella vermiformis and Willaertia magna

“…To monitor the extracellular growth of L. pneumophila strains, legionellae grown on BCYE agar were inoculated into buffered yeast extract (BYE) broth and then incubated at 37 uC with shaking (Chatfield et al, 2011). The optical densities of the cultures were determined at 660 nm using a DU720 spectrophotometer (Beckman Coulter).…”