The cytochrome c maturation locus of Legionella pneumophila promotes iron assimilation and intracellular infection and contains a strain-specific insertion sequence element

Multiple major disease-associated clones of Legionella pneumophila have emerged recently and independently

Legionella pneumophila is an environmental bacterium and the leading cause of Legionnaires' disease. Just five sequence types (ST), from more than 2000 currently described, cause nearly half of disease cases in northwest Europe. Here, we report the sequence and analyses of 364 L. pneumophila genomes, including 337 from the five disease-associated STs and 27 representative of the species diversity. Phylogenetic analyses revealed that the five STs have independent origins within a highly diverse species. The number of de novo mutations is extremely low with maximum pairwise single-nucleotide polymorphisms (SNPs) ranging from 19 (ST47) to 127 (ST1), which suggests emergences within the last century. Isolates sampled geographically far apart differ by only a few SNPs, demonstrating rapid dissemination. These five STs have been recombining recently, leading to a shared pool of allelic variants potentially contributing to their increased disease propensity. The oldest clone, ST1, has spread globally; between 1940 and 2000, four new clones have emerged in Europe, which show long-distance, rapid dispersal. That a large proportion of clinical cases is caused by recently emerged and internationally dispersed clones, linked by convergent evolution, is surprising for an environmental bacterium traditionally considered to be an opportunistic pathogen. To simultaneously explain recent emergence, rapid spread and increased disease association, we hypothesize that these STs have adapted to new man-made environmental niches, which may be linked by human infection and transmission.

Iron Limitation Triggers Early Egress by the Intracellular Bacterial Pathogen Legionella pneumophila

Legionella pneumophila is an intracellular bacterial pathogen that replicates in alveolar macrophages, causing a severe form of pneumonia. Intracellular growth of the bacterium depends on its ability to sequester iron from the host cell. In the L. pneumophila strain 130b, one mechanism used to acquire this essential nutrient is the siderophore legiobactin. Iron-bound legiobactin is imported by the transport protein LbtU. Here, we describe the role of LbtP, a paralog of LbtU, in iron acquisition in the L. pneumophila strain Philadelphia-1. Similar to LbtU, LbtP is a siderophore transport protein and is required for robust growth under iron-limiting conditions. Despite their similar functions, however, LbtU and LbtP do not contribute equally to iron acquisition. The Philadelphia-1 strain lacking LbtP is more sensitive to iron deprivation in vitro Moreover, LbtP is important for L. pneumophila growth within macrophages while LbtU is dispensable. These results demonstrate that LbtP plays a dominant role over LbtU in iron acquisition. In contrast, loss of both LbtP and LbtU does not impair L. pneumophila growth in the amoebal host Acanthamoeba castellanii, demonstrating a host-specific requirement for the activities of these two transporters in iron acquisition. The growth defect of the ΔlbtP mutant in macrophages is not due to alterations in growth kinetics. Instead, the absence of LbtP limits L. pneumophila replication and causes bacteria to prematurely exit the host cell. These results demonstrate the existence of a preprogrammed exit strategy in response to iron limitation that allows L. pneumophila to abandon the host cell when nutrients are exhausted.

An update on iron acquisition by Legionella pneumophila: new pathways for siderophore uptake and ferric iron reduction

Iron acquisition is critical for the growth and pathogenesis of Legionella pneumophila, the causative agent of Legionnaires' disease. L. pneumophila utilizes two main modes of iron assimilation, namely ferrous iron uptake via the FeoB system and ferric iron acquisition through the action of the siderophore legiobactin. This review highlights recent studies concerning the mechanism of legiobactin assimilation, the impact of c-type cytochromes on siderophore production, the importance of legiobactin in lung infection and a newfound role for a bacterial pyomelanin in iron acquisition. These data demonstrate that key aspects of L. pneumophila iron acquisition are significantly distinct from those of long-studied, 'model' organisms. Indeed, L. pneumophila may represent a new paradigm for a variety of other intracellular parasites, pathogens and under-studied bacteria.

Different distribution patterns of ten virulence genes in Legionella reference strains and strains isolated from environmental water and patients

Virulence genes are distinct regions of DNA which are present in the genome of pathogenic bacteria and absent in nonpathogenic strains of the same or related species. Virulence genes are frequently associated with bacterial pathogenicity in genus Legionella. In the present study, an assay was performed to detect ten virulence genes, including iraA, iraB, lvrA, lvrB, lvhD, cpxR, cpxA, dotA, icmC and icmD in different pathogenicity islands of 47 Legionella reference strains, 235 environmental strains isolated from water, and 4 clinical strains isolated from the lung tissue of pneumonia patients. The distribution frequencies of these genes in reference or/and environmental L. pneumophila strains were much higher than those in reference non-L. pneumophila or/and environmental non-L. pneumophila strains, respectively. L. pneumophila clinical strains also maintained higher frequencies of these genes compared to four other types of Legionella strains. Distribution frequencies of these genes in reference L. pneumophila strains were similar to those in environmental L. pneumophila strains. In contrast, environmental non-L. pneumophila maintained higher frequencies of these genes compared to those found in reference non-L. pneumophila strains. This study illustrates the association of virulence genes with Legionella pathogenicity and reveals the possible virulence evolution of non-L. pneumophia strains isolated from environmental water.

Nutrient salvaging and metabolism by the intracellular pathogen Legionella pneumophila

The Gram-negative bacterium Legionella pneumophila is ubiquitous in freshwater environments as a free-swimming organism, resident of biofilms, or parasite of protozoa. If the bacterium is aerosolized and inhaled by a susceptible human host, it can infect alveolar macrophages and cause a severe pneumonia known as Legionnaires' disease. A sophisticated cell differentiation program equips L. pneumophila to persist in both extracellular and intracellular niches. During its life cycle, L. pneumophila alternates between at least two distinct forms: a transmissive form equipped to infect host cells and evade lysosomal degradation, and a replicative form that multiplies within a phagosomal compartment that it has retooled to its advantage. The efficient changeover between transmissive and replicative states is fundamental to L. pneumophila's fitness as an intracellular pathogen. The transmission and replication programs of L. pneumophila are governed by a number of metabolic cues that signal whether conditions are favorable for replication or instead trigger escape from a spent host. Several lines of experimental evidence gathered over the past decade establish strong links between metabolism, cellular differentiation, and virulence of L. pneumophila. Herein, we focus on current knowledge of the metabolic components employed by intracellular L. pneumophila for cell differentiation, nutrient salvaging and utilization of host factors. Specifically, we highlight the metabolic cues that are coupled to bacterial differentiation, nutrient acquisition systems, and the strategies utilized by L. pneumophila to exploit host metabolites for intracellular replication.

Protein Machineries Involved in the Attachment of Heme to Cytochrome c: Protein Structures and Molecular Mechanisms

Cytochromes c (Cyt c) are ubiquitous heme-containing proteins, mainly involved in electron transfer processes, whose structure and functions have been and still are intensely studied. Surprisingly, our understanding of the molecular mechanism whereby the heme group is covalently attached to the apoprotein (apoCyt) in the cell is still largely unknown. This posttranslational process, known as Cyt c biogenesis or Cyt c maturation, ensures the stereospecific formation of the thioether bonds between the heme vinyl groups and the cysteine thiols of the apoCyt heme binding motif. To accomplish this task, prokaryotic and eukaryotic cells have evolved distinctive protein machineries composed of different proteins. In this review, the structural and functional properties of the main maturation apparatuses found in gram-negative and gram-positive bacteria and in the mitochondria of eukaryotic cells will be presented, dissecting the Cyt c maturation process into three functional steps: (i) heme translocation and delivery, (ii) apoCyt thioreductive pathway, and (iii) apoCyt chaperoning and heme ligation. Moreover, current hypotheses and open questions about the molecular mechanisms of each of the three steps will be discussed, with special attention to System I, the maturation apparatus found in gram-negative bacteria.

The major facilitator superfamily-type protein LbtC promotes the utilization of the legiobactin siderophore by Legionella pneumophila

The Gram-negative bacterium Legionella pneumophila elaborates the siderophore legiobactin. We previously showed that cytoplasmic LbtA helps mediate legiobactin synthesis, inner-membrane LbtB promotes export of legiobactin, and outer-membrane LbtU acts as the ferrisiderophore receptor. RT-PCR analyses now identified lbtC as an iron-repressed gene that is the final gene in an operon containing lbtA and lbtB. In silico analysis predicted that LbtC is an inner-membrane protein that belongs to the major facilitator superfamily (MFS). Although capable of normal growth in standard media, lbtC mutants were defective for growth on iron-depleted agar media. While producing normal levels of legiobactin, lbtC mutants were unable to utilize supplied legiobactin to stimulate growth on iron-depleted media and displayed an impaired ability to take up radiolabelled iron. All lbtC mutant phenotypes were complemented by reintroduction of an intact copy of lbtC. When a cloned copy of both lbtC and lbtU was introduced into a heterologous bacterium (Legionella longbeachae), the organism acquired the ability to utilize legiobactin to grow better on low-iron media. Together, these data indicate that LbtC is involved in the uptake of legiobactin, and based upon its predicted location is most likely the mediator of ferrilegiobactin transport across the inner membrane. The data are also a unique documentation of how an MFS protein can promote bacterial iron-siderophore import, standing in contrast to the vast majority of studies which have defined ABC-type permeases as the mediators of siderophore import across the Gram-negative inner membrane or the Gram-positive cytoplasmic membrane.

Intra-Vacuolar Proliferation of F. Novicida within H. Vermiformis

Francisella tularensis is a gram negative facultative intracellular bacterium that causes the zoonotic disease tularemia. Free-living amebae, such as Acanthamoeba and Hartmannella, are environmental hosts of several intracellular pathogens. Epidemiology of F. tularensis in various parts of the world is associated with water-borne transmission, which includes mosquitoes and amebae as the potential host reservoirs of the bacteria in water resources. In vitro studies showed intracellular replication of F. tularensis within A. castellanii cells. Whether ameba is a biological reservoir for Francisella in the environment is not known. We used Hartmannella vermiformis as an amebal model system to study the intracellular life of F. novicida. For the first time we show that F. novicida survives and replicates within H. vermiformis. The iglC mutant strain of F. novicida is defective for survival and replication not only within A. castellanii but also in H. vermiformis cells. In contrast to mammalian cells, where bacteria replicate in the cytosol, F. novicida resides and replicates within membrane-bound vacuoles within the trophozoites of H. vermiformis. In contrast to the transient residence of F. novicida within acidic vacuoles prior to escaping to the cytosol of mammalian cells, F. novicida does not reside transiently or permanently in an acidic compartment within H. vermiformis when examined 30 min after initiation of the infection. We conclude that F. tularensis does not replicate within acidified vacuoles and does not escape into the cytosol of H. vermiformis. The Francisella pathogenicity island locus iglC is essential for intra-vacuolar proliferation of F. novicida within H. vermiformis. Our data show a distinct intracellular lifestyle for F. novicida within H. vermiformis compared to mammalian cells.

DsbA2 (27 kDa Com1-like protein) of Legionella pneumophila catalyses extracytoplasmic disulphide-bond formation in proteins including the Dot/Icm type IV secretion system

In Gram-negative bacteria, thiol oxidoreductases catalyse the formation of disulphide bonds (DSB) in extracytoplasmic proteins. In this study, we sought to identify DSB-forming proteins required for assembly of macromolecular structures in Legionella pneumophila. Here we describe two DSB-forming proteins, one annotated as dsbA1 and the other annotated as a 27 kDa outer membrane protein similar to Com1 of Coxiella burnetii, which we designate as dsbA2. Both proteins are predicted to be periplasmic, and while dsbA1 mutants were readily isolated and without phenotype, dsbA2 mutants were not obtained. To advance studies of DsbA2, a cis-proline residue at position 198 was replaced with threonine that enables formation of stable disulphide-bond complexes with substrate proteins. Expression of DsbA2 P198T mutant protein from an inducible promoter produced dominant-negative effects on DsbA2 function that resulted in loss of infectivity for amoeba and HeLa cells and loss of Dot/Icm T4SS-mediated contact haemolysis of erythrocytes. Analysis of captured DsbA2 P198T-substrate complexes from L. pneumophila by mass spectrometry identified periplasmic and outer membrane proteins that included components of the Dot/Icm T4SS. More broadly, our studies establish a DSB oxidoreductase function for the Com1 lineage of DsbA2-like proteins which appear to be conserved among those bacteria also expressing T4SS.

Legionella pneumophila LbtU acts as a novel, TonB-independent receptor for the legiobactin siderophore

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 β-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 β-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.

Cytochrome c4 is required for siderophore expression by Legionella pneumophila, whereas cytochromes c1 and c5 promote intracellular infection

A panel of cytochrome c maturation (ccm) mutants of Legionella pneumophila displayed a loss of siderophore (legiobactin) expression, as measured by both the chrome azurol S assay and a Legionella-specific bioassay. These data, coupled with the finding that ccm transcripts are expressed by wild-type bacteria grown in deferrated medium, indicate that the Ccm system promotes siderophore expression by L. pneumophila. To determine the basis of this newfound role for Ccm, we constructed and tested a set of mutants specifically lacking individual c-type cytochromes. Whereas ubiquinol-cytochrome c reductase (petC) mutants specifically lacking cytochrome c₁ and cycB mutants lacking cytochrome c₅ had normal siderophore expression, cyc4 mutants defective for cytochrome c₄ completely lacked legiobactin. These data, along with the expression pattern of cyc4 mRNA, indicate that cytochrome c₄ in particular promotes siderophore expression. In intracellular infection assays, petC mutants and cycB mutants, but not cyc4 mutants, had a reduced ability to infect both amoebae and macrophage hosts. Like ccm mutants, the cycB mutants were completely unable to grow in amoebae, highlighting a major role for cytochrome c₅ in intracellular infection. To our knowledge, these data represent both the first direct documentation of the importance of a c-type cytochrome in expression of a biologically active siderophore and the first insight into the relative importance of c-type cytochromes in intracellular infection events.

Molecular pathogenesis of infections caused by Legionella pneumophila

The genus Legionella contains more than 50 species, of which at least 24 have been associated with human infection. The best-characterized member of the genus, Legionella pneumophila, is the major causative agent of Legionnaires' disease, a severe form of acute pneumonia. L. pneumophila is an intracellular pathogen, and as part of its pathogenesis, the bacteria avoid phagolysosome fusion and replicate within alveolar macrophages and epithelial cells in a vacuole that exhibits many characteristics of the endoplasmic reticulum (ER). The formation of the unusual L. pneumophila vacuole is a feature of its interaction with the host, yet the mechanisms by which the bacteria avoid classical endosome fusion and recruit markers of the ER are incompletely understood. Here we review the factors that contribute to the ability of L. pneumophila to infect and replicate in human cells and amoebae with an emphasis on proteins that are secreted by the bacteria into the Legionella vacuole and/or the host cell. Many of these factors undermine eukaryotic trafficking and signaling pathways by acting as functional and, in some cases, structural mimics of eukaryotic proteins. We discuss the consequences of this mimicry for the biology of the infected cell and also for immune responses to L. pneumophila infection.

A type II secreted RNase of Legionella pneumophila facilitates optimal intracellular infection of Hartmannella vermiformis

Type II protein secretion plays a role in a wide variety of functions that are important for the ecology and pathogenesis of Legionella pneumophila. Perhaps most dramatic is the critical role that this secretion pathway has in L. pneumophila intracellular infection of aquatic protozoa. Recently, we showed that virulent L. pneumophila strain 130b secretes RNase activity through its type II secretion system. We now report the cloning and mutational analysis of the gene (srnA) encoding that novel type of secreted activity. The SrnA protein was defined as being a member of the T2 family of secreted RNases. Supernatants from mutants inactivated for srnA completely lacked RNase activity, indicating that SrnA is the major secreted RNase of L. pneumophila. Although srnA mutants grew normally in bacteriological media and human U937 cell macrophages, they were impaired in their ability to grow within Hartmannella vermiformis amoebae. This finding represents the second identification of a L. pneumophila type II effector being necessary for optimal intracellular infection of amoebae, with the first being the ProA zinc metalloprotease. Newly constructed srnA proA double mutants displayed an even larger infection defect that appeared to be the additive result of losing both SrnA and ProA. Overall, these data represent the first demonstration of a secreted RNase promoting an intracellular infection event, and support our long-standing hypothesis that the infection defects of L. pneumophila type II secretion mutants are due to the loss of multiple secreted effectors.

Functional analysis of the multi-copper oxidase from Legionella pneumophila

Multicopper oxidases have been described to have functions in copper tolerance, manganese oxidation, and iron oxidation in a range of bacteria. The putative cytoplasmic membrane multicopper oxidase from Legionella pneumophila was investigated. The mcoL gene was found to be critical for aerobic extracellular growth under either iron-limiting conditions or in the presence of ferrous Fe(II) iron, as a sole source of this essential metal. The mcoL mutants showed minor growth defects when grown in the presence of Fe(III) as the iron source. In contrast, intracellular growth and survival was not affected by the absence of the mcoL gene regardless of available iron concentration. The evidence presented here could indicate a possible role for mcoL in prevention of the toxic effects of ferrous iron during aerobic conditions. However, a function in high-affinity acquisition of iron could also be possible given the inability of the McoL mutants to grow aerobically under iron-limiting conditions.

The type II secretion system of Legionella pneumophila elaborates two aminopeptidases, as well as a metalloprotease that contributes to differential infection among protozoan hosts

Legionella pneumophila, the agent of Legionnaires' disease, is an intracellular parasite of aquatic amoebae and human macrophages. A key factor for L. pneumophila in intracellular infection is its type II protein secretion system (Lsp). In order to more completely define Lsp output, we recently performed a proteomic analysis of culture supernatants. Based upon the predictions of that analysis, we found that L. pneumophila secretes two distinct aminopeptidase activities encoded by the genes lapA and lapB. Whereas lapA conferred activity against leucine, phenylalanine, and tyrosine aminopeptides, lapB was linked to the cleavage of lysine- and arginine-containing substrates. To assess the role of secreted aminopeptidases in intracellular infection, we examined the relative abilities of lapA and lapB mutants to infect human U937 cell macrophages as well as Hartmannella vermiformis and Acanthamoeba castellanii amoebae. Although these experiments identified a dispensable role for LapA and LapB, they uncovered a previously unrecognized role for the type II-dependent ProA (MspA) metalloprotease. Whereas proA mutants were not defective for macrophage or A. castellanii infection, they (but not their complemented derivatives) were impaired for growth upon coculture with H. vermiformis. Thus, ProA represents the first type II effector implicated in an intracellular infection event. Furthermore, proA represents an L. pneumophila gene that shows differential importance among protozoan infection models, suggesting that the legionellae might have evolved some of its factors to especially target certain of their protozoan hosts.

Presence in Legionella pneumophila of a mammalian-like mitochondrial permeability transition pore?

The genome of Legionella pneumophila reveals the presence of a large number of genes coding for eukaryotic-like proteins. By using database searches and homology investigations, we identified three proteins in L. pneumophila whose sequences share similarities with that of eukaryotic polypeptides (lpg0211, lpg1974 and lpg1982). In eukaryotes, the corresponding proteins (PBR, peripheral benzodiazepine receptor; VDAC, voltage-dependant anion channel; and CypD, cyclophilin D) participate in the formation of the mammalian mitochondrial permeability transition pore (MPTP), a complex involved in cell apoptosis. Intriguingly, the presence of these proteins has never been reported in the same bacterium and constitutes, up to now, a unique feature of L. pneumophila. In Legionella, we hypothesize that these proteins are recruited in a multiprotein complex close to the MPTP that may regulate intracellular survival and/or proliferation.

The secreted pyomelanin pigment of Legionella pneumophila confers ferric reductase activity

The virulence of Legionella pneumophila is dependent upon its capacity to acquire iron. To identify genes involved in expression of its siderophore, we screened a mutagenized population of L. pneumophila for strains that were no longer able to rescue the growth of a ferrous transport mutant. However, an unusual mutant was obtained that displayed a strong inhibitory effect on the feoB mutant. Due to an insertion in hmgA that encodes homogentisate 1,2-dioxygenase, the mutant secreted increased levels of pyomelanin, the L. pneumophila pigment that is derived from secreted homogentisic acid (HGA). Thus, we hypothesized that L. pneumophila-secreted HGA-melanin has intrinsic ferric reductase activity, converting Fe(3+) to Fe(2+), but that hyperpigmentation results in excessive reduction of iron that can, in the case of the feoB mutant, be inhibitory to growth. In support of this hypothesis, we demonstrated, for the first time, that wild-type L. pneumophila secretes ferric reductase activity. Moreover, whereas the hyperpigmented mutant had increased secreted activity, an lly mutant specifically impaired for pigment production lacked the activity. Compatible with the nature of HGA-melanins, the secreted ferric reductase activity was positively influenced by the amount of tyrosine in the growth medium, resistant to protease, acid precipitable, and heterogeneous in size. Together, these data represent the first demonstration of pyomelanin-mediated ferric reduction by a pathogenic bacterium.

A conserved histidine in cytochrome c maturation permease CcmB of Shewanella putrefaciens is required for anaerobic growth below a threshold standard redox potential

Shewanella putrefaciens strain 200 respires a wide range of compounds as terminal electron acceptor. The respiratory versatility of Shewanella is attributed in part to a set of c-type cytochromes with widely varying midpoint redox potentials (E'(0)). A point mutant of S. putrefaciens, originally designated Urr14 and here renamed CCMB1, was found to grow at wild-type rates on electron acceptors with high E'0 [O2, NO3-, Fe(III) citrate, MnO2, and Mn(III) pyrophosphate] yet was severely impaired for growth on electron acceptors with low E'0 [NO2-, U(VI), dimethyl sulfoxide, TMAO (trimethylamine N-oxide), fumarate, gamma-FeOOH, SO3(2-), and S2O3(2-)]. Genetic complementation and nucleotide sequence analyses indicated that the CCMB1 respiratory mutant phenotype was due to mutation of a conserved histidine residue (H108Y) in a protein that displayed high homology to Escherichia coli CcmB, the permease subunit of an ABC transporter involved in cytochrome c maturation. Although CCMB1 retained the ability to grow on electron acceptors with high E'(0), the cytochrome content of CCMB1 was <10% of that of the wild-type strain. Periplasmic extracts of CCMB1 contained slightly greater concentrations of the thiol functional group (-SH) than did the wild-type strain, an indication that the E(h) of the CCMB1 periplasm was abnormally low. A ccmB deletion mutant was unable to respire anaerobically on any electron acceptor, yet retained aerobic respiratory capability. These results suggest that the mutation of a conserved histidine residue (H108) in CCMB1 alters the redox homeostasis of the periplasm during anaerobic growth on electron acceptors with low (but not high) E'0. This is the first report of the effects of Ccm deficiencies on bacterial respiration of electron acceptors whose E'0 nearly span the entire redox continuum.

Iron acquisition by Legionella pneumophila

For nearly 20 years, it was believed that Legionella pneumophila does not produce siderophores. Yet, we have now determined that L. pneumophila secretes a siderophore (legiobactin) that is detectable by the CAS assay. We have optimized conditions for legiobactin expression, shown its biological activity, and found genes (lbtAB) involved in its production and secretion. LbtA is homologous with siderophore synthetases from E. coli (aerobactin), Sinorhizobium (rhizobactin), and Bordetella (alcaligin), while LbtB is a member of the major facilitator superfamily of multidrug efflux pumps. Mutants lacking lbtAB produce 40-70% less CAS reactivity. The lbtA mutant is also defective for growth in deferrated media containing citrate, indicating that legiobactin is required in conditions of severe iron limitation. lbtAB mutants grow normally in macrophages and amoebae host cells as well as within the lungs of mice. L. pneumophila does express lbtA in macrophages, suggesting that legiobactin has a dispensable role in infection. Legiobactin is iron repressed and does not react in the Csáky and Arnow assays. Anion-exchange HPLC has been used to purify legiobactin, and thus far, structural analysis suggests that the molecule is similar but not identical to rhizobactin, rhizoferrin, and alcaligin. The residual CAS reactivity present in supernatants of the lbtAB mutants suggests that L. pneumophila might produce a second siderophore. Besides siderophores, we have determined that ferrous iron transport, encoded by feoB, is critical for L. pneumophila growth in low-iron conditions, in host cells, and in the mammalian lung. Some of our other studies have discovered a critical, yet undefined, role for the L. pneumophila cytochrome c maturation locus in low-iron growth, intracellular infection, and virulence.

Heme concentration dependence and metalloporphyrin inhibition of the system I and II cytochrome c assembly pathways

Studies have indicated that specific heme delivery to apocytochrome c is a critical feature of the cytochrome c biogenesis pathways called system I and II. To determine directly the heme requirements of each system, including whether other metal porphyrins can be incorporated into cytochromes c, we engineered Escherichia coli so that the natural system I (ccmABCDEFGH) was deleted and exogenous porphyrins were the sole source of porphyrins (Delta hemA). The engineered E. coli strains that produced recombinant system I (from E. coli) or system II (from Helicobacter) facilitated studies of the heme concentration dependence of each system. Using this exogenous porphyrin approach, it was shown that in system I the levels of heme used are at least fivefold lower than the levels used in system II, providing an important advantage for system I. Neither system could assemble holocytochromes c with other metal porphyrins, suggesting that the attachment mechanism is specific for Fe protoporphyrin. Surprisingly, Zn and Sn protoporphyrins are potent inhibitors of the pathways, and exogenous heme competes with this inhibition. We propose that the targets are the heme binding proteins in the pathways (CcmC, CcmE, and CcmF for system I and CcsA for system II).

Distribution of 19 major virulence genes in Legionella pneumophila serogroup 1 isolates from patients and water in Queensland, Australia

The distribution of 19 major virulence genes and the presence of plasmids were surveyed in 141 Legionella pneumophila serogroup (SG) 1 isolates from patients and water in Queensland, Australia. The results showed that 16 of the virulence genes examined were present in all isolates, suggesting that they are life-essential genes for isolates in the environment and host cells. The 65 kb pathogenicity island identified originally in strain Philadelphia-1(T) was detected more frequently in isolates from water (44.2%) than in those from patients (2.7%), indicating that the 65 kb DNA fragment may aid the survival of L. pneumophila in the sampled environment. However, the low frequency of the 65 kb fragment in isolates from patients suggests that the pathogenicity island may not be necessary for L. pneumophila to cause disease. Plasmids were not detected in the L. pneumophila SG1 isolates from patients or water studied. There was an association of both lvh and rtxA with the virulent and predominant genotype detected by amplified fragment length polymorphism, termed AF1, whereas the avirulent common isolate from water termed AF16 did not have lvh or rtxA genes, with the exception of one isolate with rtxA. It was found that a PCR detection test strategy with lvh and rtxA as pathogenesis markers would be useful for determining the infection potential of an isolate.

ABC transporter-mediated release of a haem chaperone allows cytochromecbiogenesis

Although organisms from all kingdoms have either the system I or II cytochrome c biogenesis pathway, it has remained a mystery as to why these two distinct pathways have developed. We have previously shown evidence that the system I pathway has a higher affinity for haem than system II for cytochrome c biogenesis. Here, we show the mechanism by which the system I pathway can utilize haem at low levels. The mechanism involves an ATP-binding cassette (ABC) transporter that is required for release of the periplasmic haem chaperone CcmE to the last step of cytochrome c assembly. This ABC transporter is composed of the ABC subunit CcmA, and two membrane proteins, CcmB and CcmC. In the absence of CcmA or CcmB, holo(haem)CcmE binds to CcmC in a stable dead-end complex, indicating high affinity binding of haem to CcmC. Expression of CcmA and CcmB facilitates formation of the CcmA2B1C1 complex and ATP-dependent release of holoCcmE. We propose that the CcmA2B1C1 complex represents a new subgroup within the ABC transporter superfamily that functions to release a chaperone.

lbtA and lbtB are required for production of the Legionella pneumophila siderophore legiobactin

Under iron stress, Legionella pneumophila secretes legiobactin, a nonclassical siderophore that is reactive in the chrome azurol S (CAS) assay. Here, we have optimized conditions for legiobactin expression, shown its biological activity, and identified two genes, lbtA and lbtB, which are involved in legiobactin production. lbtA appears to be iron repressed and encodes a protein that has significant homology with siderophore synthetases, and FrgA, a previously described iron-regulated protein of L. pneumophila. lbtB encodes a protein homologous with members of the major facilitator superfamily of multidrug efflux pumps. Mutants lacking lbtA or lbtB were defective for legiobactin, producing 40 to 70% less CAS reactivity in deferrated chemically defined medium (CDM). In bioassays, mutant CDM culture supernatants, unlike those of the wild type, did not support growth of iron-limited wild-type bacteria in 2',2'-dipyridyl-containing buffered charcoal yeast extract (BCYE) agar and a ferrous iron transport mutant on BCYE agar without added iron. The lbtA mutant was modestly defective for growth in deferrated CDM containing the iron chelator citrate, indicating that legiobactin is required in conditions of severe iron limitation. Complementation of the lbt mutants restored both siderophore expression, as measured by the CAS assay and bioassays, and bacterial growth in deferrated, citrate-containing media. The lbtA mutant replicated as the wild type did in macrophages, amoebae, and the lungs of mice. However, L. pneumophila expresses lbtA in the macrophage, suggesting that legiobactin, though not required, may play a dispensable role in intracellular growth. The discovery of lbtAB represents the first identification of genes required for L. pneumophila siderophore expression.

Impact of the bacterial type I cytochromecmaturation system on different biological processes

In the alpha-, beta- and gamma-Proteobacteria, the so-called cytochrome c maturation (Ccm) system is known to promote the covalent attachment of the haem to periplasmic apocytochrome c. However, in species of Pseudomonas, Rhizobium, Paracoccus and Legionella, mutations in ccm genes result in phenotypes that cannot be readily explained by the simple loss of a c-type cytochrome. These phenotypes include loss of siderophore production and utilization, reduced abilities to grow in low-iron conditions and in mammalian and protozoan host cells, and alterations in copper sensitivity and manganese oxidation. These various data suggest that Ccm proteins may perform one or more functions in addition to Ccm, which are critical for bacterial physiology and growth. Novel hypotheses that should be explored include the utilization of Ccm-associated haem for processes besides attachment to apocytochrome c, the export of a non-haem compound through the Ccm system, and the negative effects of protoporphyrin IX accumulation.

Cytochrome c maturation proteins are critical for in vivo growth of Legionella pneumophila

Legionella pneumophila, an intracellular parasite of macrophages and protozoa, requires iron for extra- and intracellular growth. In a new screen of a mutant library of L. pneumophila for strains defective for growth on agar media lacking supplemental iron, seven mutants were obtained. All of the mutants had a disruption in the cytochrome c maturation (ccm) locus; two had insertions in ccmB, two in ccmC, and three in ccmF. The ccm mutants were unable to multiply within macrophage-like cells (i.e., U937 and THP-1 cells) and Hartmannella vermiformis amoebae. A competition assay in A/J mice revealed that ccm mutants are severely defective for growth within the lung. Taken together, these data confirm that ccm and cytochrome c maturation proteins are required for L. pneumophila growth in low iron, intracellular infection, and virulence.

The Legionella pneumophila tatB gene facilitates secretion of phospholipase C, growth under iron-limiting conditions, and intracellular infection

Our previous mutational analysis of Legionella pneumophila demonstrated a role for type II protein (Lsp) secretion and iron acquisition in intracellular infection and virulence. In gram-negative bacteria, the twin-arginine translocation (Tat) pathway is involved in secretion of proteins, including components of respiratory complexes, across the inner membrane to the periplasm. To assess the significance of Tat for L. pneumophila, tatB mutants were characterized. The mutants exhibited normal growth in standard media but grew slowly under low-iron conditions. They were also impaired in the Nadi assay, indicating that the function of cytochrome c oxidase is influenced by tatB. Consistent with this observation, a subunit of the cytochrome c reductase was shown to be a Tat substrate. Supernatants of the tatB mutants showed a 30% reduction in phospholipase C activity while maintaining normal levels of other Lsp secreted activities. When tested for infection of U937 macrophages, the tatB mutants showed a 10-fold reduction in growth. Double mutants lacking tatB and Lsp secretion were even more defective, suggesting tatB has an intracellular role that is independent of Lsp. tatB mutants were also impaired 20-fold in Hartmannella vermiformis amoebae cultured in the presence of an iron chelator. All mutant phenotypes were complemented by reintroduction of an intact tatB. Thus, L. pneumophila tatB plays a role in the formation of a respiratory complex, growth under low-iron conditions, the secretion of a phospholipase C activity, and intracellular infection.

Indole-3-acetic acid biosynthesis is deficient in Gluconacetobacter diazotrophicus strains with mutations in cytochrome c biogenesis genes

Gluconacetobacter diazotrophicus is an endophyte of sugarcane frequently found in plants grown in agricultural areas where nitrogen fertilizer input is low. Recent results from this laboratory, using mutant strains of G. diazotrophicus unable to fix nitrogen, suggested that there are two beneficial effects of G. diazotrophicus on sugarcane growth: one dependent and one not dependent on nitrogen fixation. A plant growth-promoting substance, such as indole-3-acetic acid (IAA), known to be produced by G. diazotrophicus, could be a nitrogen fixation-independent factor. One strain, MAd10, isolated by screening a library of Tn5 mutants, released only approximately 6% of the amount of IAA excreted by the parent strain in liquid culture. The mutation causing the IAA(-) phenotype was not linked to Tn5. A pLAFR3 cosmid clone that complemented the IAA deficiency was isolated. Sequence analysis of a complementing subclone indicated the presence of genes involved in cytochrome c biogenesis (ccm, for cytochrome c maturation). The G. diazotrophicus ccm operon was sequenced; the individual ccm gene products were 37 to 52% identical to ccm gene products of Escherichia coli and equivalent cyc genes of Bradyrhizobium japonicum. Although several ccm mutant phenotypes have been described in the literature, there are no reports of ccm gene products being involved in IAA production. Spectral analysis, heme-associated peroxidase activities, and respiratory activities of the cell membranes revealed that the ccm genes of G. diazotrophicus are involved in cytochrome c biogenesis.

Co-ordination of iron acquisition, iron porphyrin chelation and iron-protoporphyrin export via the cytochrome c biogenesis protein CcmC in Pseudomonas fluorescens

The cytoplasmic membrane protein CcmC is, together with other Ccm proteins, a component for the maturation of c-type cytochromes in Gram-negative bacteria. A Pseudomonas fluorescens ATCC 17400 ccmC mutant is cytochrome c-deficient and shows considerably reduced production of the two siderophores pyoverdine and quinolobactin, paralleled by a general inability to utilize various iron sources, with the exception of haem. The ccmC mutant accumulates in a 5-aminolevulinic acid-dependent synthesis a reddish, fluorescent pigment identified as protoporphyrin IX. As a consequence a visA phenotype similar to that of a ferrochelatase-deficient hemH mutant characterized by drastically reduced growth upon light exposure was observed for the ccmC mutant. The defect of iron-protoporphyrin formation was further demonstrated by the failure of ccmC cell-free proteinase K-treated extracts to stimulate the growth of a haem auxotrophic hemH indicator strain, compared to similarly prepared wild-type extracts. In addition, the ccmC mutant did not sustain hemH growth in cross-feeding experiments while the wild-type did. Significantly reduced resistance to oxidative stress mediated by haem-containing catalases was observed for the ccmC mutant. A double hemH ccmC mutant could not be obtained in the presence of external haem without the hemH gene in trans, indicating that the combination of the two mutations is lethal. It was concluded that CcmC, apart from its known function in cytochrome c biogenesis, plays a role in haem biosynthesis. A function in the regulatory co-ordination of iron acquisition via siderophores, iron insertion into porphyrin via ferrochelatase and iron-protoporphyrin export for cytochrome c formation is predicted.

Legionella pneumophila type II protein secretion promotes virulence in the A/J mouse model of Legionnaires' disease pneumonia

Legionella pneumophila, the gram-negative agent of Legionnaires' disease, possesses type IV pili and a type II protein secretion (Lsp) system, both of which are dependent upon the PilD prepilin peptidase. By analyzing multiple pilD mutants and various types of Lsp mutants as well as performing trans-complementation of these mutants, we have confirmed that PilD and type II secretion genes are required for L. pneumophila infection of both amoebae and human macrophages. Based upon a complete analysis of lspDE, lspF, and lspG mutants, we found that the type II system controls the secretion of protease, RNase, lipase, phospholipase A, phospholipase C, lysophospholipase A, and tartrate-sensitive and tartrate-resistant acid phosphatase activities and influences the appearance of colonies. Examination of the developing L. pneumophila genome database indicated that the organism has two other loci (lspC and lspLM) that are predicted to promote secretion and thus a set of genes that is comparable to the type II secretion genes in other gram-negative bacteria. In contrast to lsp mutants, L. pneumophila pilus mutants lacking either the PilQ secretin, the PspA pseudopilin, or pilin were not defective for colonial growth, secreted activities, or intracellular replication. L. pneumophila dot/icm mutants were also not impaired for type II-dependent exoenzymes. Upon intratracheal inoculation into A/J mice, lspDE, lspF, and pilD mutants, but not pilus mutants, exhibited a reduced ability to grow in the lung, as measured by competition assays. The lspF mutant was also defective in an in vivo kinetic assay. Examination of infected mouse sera revealed that type II secreted proteins are expressed in vivo. Thus, the L. pneumophila Lsp system is a virulence factor and the only type II secretion system linked to intracellular infection.

Dynamic features of a heme delivery system for cytochrome C maturation

In Escherichia coli, heme is delivered to cytochrome c in a process involving eight proteins encoded by the ccmABCDEFGH operon. Heme is transferred to the periplasmic heme chaperone CcmE by CcmC and from there to apocytochrome c. The role of CcmC was investigated by random as well as site-directed mutagenesis. Important amino acids were all located in periplasmic domains of the CcmC protein that has six membrane-spanning helices. Besides the tryptophan-rich motif and two conserved histidines, new residues were identified as functionally important. Mutants G111S and H184Y had a clear defect in CcmC-CcmE interaction, did not transfer heme to CcmE, and lacked c-type cytochromes. Conversely, mutants D47N, R55P, and S176Y were affected neither in interaction with nor in delivery of heme to CcmE but produced less than 10% c-type cytochromes. A strain carrying a CcmCE fusion had a similar phenotype, suggesting that CcmC is important not only for heme transfer to CcmE but also for its delivery to cytochrome c. Co-immunoprecipitation of CcmC with CcmF was not detectable although CcmE co-precipitated individually with CcmC and CcmF. This contradicts the idea of CcmCEF supercomplex formation. Our results favor a model that predicts CcmE to shuttle between CcmC and CcmF for heme delivery.

Chemiluminescent-based methods to detect subpicomole levels of c-type cytochromes

A variety of luminol-based substrates and either an autoradiographic film or a charge-coupled device (CCD) imaging system were used for chemiluminescence detection of c-type cytochromes. The Pierce Femto peroxidase substrate was at least 10 times more sensitive when using film than the highly cited 3,3('),5,5(')-tetramethylbenzidine (benzidine derivative) staining method and 50 times more sensitive when using a CCD imaging system. Film or CCD imaging result in highly sensitive and quantitative signals. The quantitative detection of c-type cytochromes from single colonies or from less than 1ml of a bacterial culture is possible.

Legionella pneumophila feoAB promotes ferrous iron uptake and intracellular infection

In order to determine the role of ferrous iron transport in Legionella pathogenesis, we identified and mutated the feoB gene in virulent Legionella pneumophila strain 130b. As it is in Escherichia coli, the L. pneumophila feoB gene was contained within a putative feoAB operon. L. pneumophila feoB insertion mutants exhibited decreased ferrous but not ferric iron uptake compared to the wild type. Growth on standard buffered charcoal yeast extract agar or buffered yeast extract broth was unaffected by the loss of L. pneumophila FeoB. However, the L. pneumophila feoB mutant had a reduced ability to grow on buffered charcoal yeast extract agar with a reduced amount of its usual iron supplementation, a phenotype that could be complemented by the addition of feoB in trans. In unsupplemented buffered yeast extract broth, the feoB mutant also had a growth defect, which was further exacerbated by the addition of the ferrous iron chelator, 2,2'-dipyridyl. The feoB mutant was also 2.5 logs more resistant to streptonigrin than wild-type 130b, confirming its decreased ability to acquire iron during extracellular growth. Decreased replication of the feoB mutant was noted within iron-depleted Hartmannella vermiformis amoebae and human U937 cell macrophages. The reduced intracellular infectivity of the feoB mutant was complemented by the introduction of a plasmid containing feoAB. The L. pneumophila feoB gene conferred a modest growth advantage for the wild type over the mutant in a competition assay within the lungs of A/J mice. Taken together, these results indicate that L. pneumophila FeoB is a ferrous iron transporter that is important for extracellular and intracellular growth, especially in iron-limited environments. These data represent the first evidence for the importance of ferrous iron transport for intracellular replication by a human pathogen.