Small Regulatory RNA and Legionella pneumophila

Legionnaires' Disease in China Caused by Legionella pneumophila Corby

Legionella pneumophila is an intracellular pathogen causing pneumonia in humans. In February 2022, Legionnaires' disease caused by L. pneumophila strain Corby in a patient with lung adenocarcinoma was identified for the first time in China. This paper includes the case report and phenotypic and genomic analysis of the Corby (ICDC) strain. Its biological characteristics were evaluated by antibiotic sensitivity testing and cytology experiments, and genomic analysis was performed to understand its genetic evolution. The patient's clinical manifestations included cough, fever, pulmonary infiltration, and significantly decreased activity endurance. After empirical antimicrobial therapy, infection indicators decreased. The Corby (ICDC) strain was susceptible to nine antibiotics and exhibited strong intracellular proliferation ability. A phylogenetic tree showed that the Corby (ICDC) strain was closely related to the Corby strain, but under the pressure of a complex environment, its genome had undergone more rearrangement and inversion. The type IF CRISPR-Cas system was identified in its genome, and spacer analysis indicated that it had been invaded by several foreign plasmids, bacteria, and viruses during evolution. Legionnaires' disease caused by L. pneumophila strain Corby may be ignored in China, and it is urgent to improve long-term monitoring and investigation of aquatic environments and patients with respiratory infections to prevent a large-scale outbreak of Legionnaires' disease.

Potential probiotic approaches to control Legionella in engineered aquatic ecosystems

Opportunistic pathogens belonging to the genus Legionella are among the most reported waterborne-associated pathogens in industrialized countries. Legionella colonize a variety of engineered aquatic ecosystems and persist in biofilms where they interact with a multitude of other resident microorganisms. In this review, we assess how some of these interactions could be used to develop a biological-driven “probiotic” control approach against Legionella. We focus on: (i) mechanisms limiting the ability of Legionella to establish and replicate within some of their natural protozoan hosts; (ii) exploitative and interference competitive interactions between Legionella and other microorganisms; and (iii) the potential of predatory bacteria and phages against Legionella. This field is still emergent, and we therefore specifically highlight research for future investigations, and propose perspectives on the feasibility and public acceptance of a potential probiotic approach.

Human macrophages utilize a wide range of pathogen recognition receptors to recognize Legionella pneumophila, including Toll-Like Receptor 4 engaging Legionella lipopolysaccharide and the Toll-like Receptor 3 nucleic-acid sensor

Cytokines made by macrophages play a critical role in determining the course of Legionella pneumophila infection. Prior murine-based modeling indicated that this cytokine response is initiated upon recognition of L. pneumophila by a subset of Toll-like receptors, namely TLR2, TLR5, and TLR9. Through the use of shRNA/siRNA knockdowns and subsequently CRISPR/Cas9 knockouts (KO), we determined that TRIF, an adaptor downstream of endosomal TLR3 and TLR4, is required for full cytokine secretion by human primary and cell-line macrophages. By characterizing a further set of TLR KO's in human U937 cells, we discerned that, contrary to the viewpoint garnered from murine-based studies, TLR3 and TLR4 (along with TLR2 and TLR5) are in fact vital to the macrophage response in the early stages of L. pneumophila infection. This conclusion was bolstered by showing that i) chemical inhibitors of TLR3 and TLR4 dampen the cytokine output of primary human macrophages and ii) transfection of TLR3 and TLR4 into HEK cells conferred an ability to sense L. pneumophila. TLR3- and TLR4-dependent cytokines promoted migration of human HL-60 neutrophils across an epithelial layer, pointing to the biological importance for the newfound signaling pathway. The response of U937 cells to L. pneumophila LPS was dependent upon TLR4, a further contradiction to murine-based studies, which had concluded that TLR2 is the receptor for Legionella LPS. Given the role of TLR3 in sensing nucleic acid (i.e., dsRNA), we utilized newly-made KO U937 cells to document that DNA-sensing by cGAS-STING and DNA-PK are also needed for the response of human macrophages to L. pneumophila. Given the lack of attention given them in the bacterial field, C-type lectin receptors were similarly examined; but, they were not required. Overall, this study arguably represents the most extensive, single-characterization of Legionella-recognition receptors within human macrophages.

Quorum sensing controls persistence, resuscitation, and virulence of Legionella subpopulations in biofilms

The water-borne bacterium Legionella pneumophila is the causative agent of Legionnaires' disease. In the environment, the opportunistic pathogen colonizes different niches, including free-living protozoa and biofilms. The physiological state(s) of sessile Legionella in biofilms and their functional consequences are not well understood. Using single-cell techniques and fluorescent growth rate probes as well as promoter reporters, we show here that sessile L. pneumophila exhibits phenotypic heterogeneity and adopts growing and nongrowing ("dormant") states in biofilms and microcolonies. Phenotypic heterogeneity is controlled by the Legionella quorum sensing (Lqs) system, the transcription factor LvbR, and the temperature. The Lqs system and LvbR determine the ratio between growing and nongrowing sessile subpopulations, as well as the frequency of growth resumption ("resuscitation") and microcolony formation of individual bacteria. Nongrowing L. pneumophila cells are metabolically active, express virulence genes and show tolerance toward antibiotics. Therefore, these sessile nongrowers are persisters. Taken together, the Lqs system, LvbR and the temperature control the phenotypic heterogeneity of sessile L. pneumophila, and these factors regulate the formation of a distinct subpopulation of nongrowing, antibiotic tolerant, virulent persisters. Hence, the biofilm niche of L. pneumophila has a profound impact on the ecology and virulence of this opportunistic pathogen.

Small RNAs as Fundamental Players in the Transference of Information During Bacterial Infectious Diseases

Communication shapes life on Earth. Transference of information has played a paramount role on the evolution of all living or extinct organisms since the appearance of life. Success or failure in this process will determine the prevalence or disappearance of a certain set of genes, the basis of Darwinian paradigm. Among different molecules used for transmission or reception of information, RNA plays a key role. For instance, the early precursors of life were information molecules based in primitive RNA forms. A growing field of research has focused on the contribution of small non-coding RNA forms due to its role on infectious diseases. These are short RNA species that carry out regulatory tasks in cis or trans. Small RNAs have shown their relevance in fine tuning the expression and activity of important regulators of essential genes for bacteria. Regulation of targets occurs through a plethora of mechanisms, including mRNA stabilization/destabilization, driving target mRNAs to degradation, or direct binding to regulatory proteins. Different studies have been conducted during the interplay of pathogenic bacteria with several hosts, including humans, animals, or plants. The sRNAs help the invader to quickly adapt to the change in environmental conditions when it enters in the host, or passes to a free state. The adaptation is achieved by direct targeting of the pathogen genes, or subversion of the host immune system. Pathogens trigger also an immune response in the host, which has been shown as well to be regulated by a wide range of sRNAs. This review focuses on the most recent host-pathogen interaction studies during bacterial infectious diseases, providing the perspective of the pathogen.

6S RNA, a Global Regulator of Transcription

6S RNA is a small RNA regulator of RNA polymerase (RNAP) that is present broadly throughout the bacterial kingdom. Initial functional studies in Escherichia coli revealed that 6S RNA forms a complex with RNAP resulting in regulation of transcription, and cells lacking 6S RNA have altered survival phenotypes. The last decade has focused on deepening the understanding of several aspects of 6S RNA activity, including (i) addressing questions of how broadly conserved 6S RNAs are in diverse organisms through continued identification and initial characterization of divergent 6S RNAs; (ii) the nature of the 6S RNA-RNAP interaction through examination of variant proteins and mutant RNAs, cross-linking approaches, and ultimately a cryo-electron microscopic structure; (iii) the physiological consequences of 6S RNA function through identification of the 6S RNA regulon and promoter features that determine 6S RNA sensitivity; and (iv) the mechanism and cellular impact of 6S RNA-directed synthesis of product RNAs (i.e., pRNA synthesis). Much has been learned about this unusual RNA, its mechanism of action, and how it is regulated; yet much still remains to be investigated, especially regarding potential differences in behavior of 6S RNAs in diverse bacteria.

6S RNA is involved in acid resistance and invasion of epithelial cells in Salmonella enterica serovar Typhimurium

AIM

Acid is an important environmental condition encountered frequently by Salmonella enterica serovar Typhimurium during its pathogenesis, but the role of small-noncoding RNAs (sRNAs) in response to acid stress is poorly understood.

METHODS

We used RNA sequencing to explore acid-responsive sRNAs in S. Typhimurium.

RESULTS

It identified that 6S RNA encoded by the ssrS was significantly upregulated at pH 3.0. The 6S RNA knockout strain showed a reduced ability to survive at pH 3.0. Additionally, genes in Salmonella pathogenicity island-1 were downregulated in the 6S RNA knockout strain. The loss of 6S RNA significantly reduced S. Typhimurium invasion ability in HeLa cells and virulence in a mouse model.

CONCLUSION

These findings demonstrate that 6S RNA plays an important role in S. Typhimurium survival under extremely acid conditions and for invasion of epithelial cells.

Suppressing the CRISPR/Cas adaptive immune system in bacterial infections

Clustered regularly interspaced short palindromic repeats (CRISPR) coupled with CRISPR-associated (Cas) proteins (CRISPR/Cas) are the adaptive immune system of eubacteria and archaebacteria. This system provides protection of bacteria against invading foreign DNA, such as transposons, bacteriophages and plasmids. Three-stage processes in this system for immunity against foreign DNAs are defined as adaptation, expression and interference. Recent studies suggested a correlation between the interfering of the CRISPR/Cas locus, acquisition of antibiotic resistance and pathogenicity island. In this review article, we demonstrate and discuss the CRISPR/Cas system's roles in interference with acquisition of antibiotic resistance and pathogenicity island in some eubacteria. Totally, these systems function as the adaptive immune system of bacteria against invading foreign DNA, blocking the acquisition of antibiotic resistance and virulence factor, detecting serotypes, indirect effects of CRISPR self-targeting, associating with physiological functions, associating with infections in humans at the transmission stage, interfering with natural transformation, a tool for genome editing in genome engineering, monitoring foodborne pathogens etc. These results showed that the CRISPR/Cas system might prevent the emergence of virulence both in vitro and in vivo. Moreover, this system was shown to be a strong selective pressure for the acquisition of antibiotic resistance and virulence factor in bacterial pathogens.

Small Regulatory RNAs of Rickettsia conorii

Small regulatory RNAs comprise critically important modulators of gene expression in bacteria, yet very little is known about their prevalence and functions in Rickettsia species. R. conorii, the causative agent of Mediterranean spotted fever, is a tick-borne pathogen that primarily infects microvascular endothelium in humans. We have determined the transcriptional landscape of R. conorii during infection of Human Microvascular Endothelial Cells (HMECs) by strand-specific RNA sequencing to identify 4 riboswitches, 13 trans-acting (intergenic), and 22 cis-acting (antisense) small RNAs (termed ‘Rc_sR’s). Independent expression of four novel trans-acting sRNAs (Rc_sR31, Rc_sR33, Rc_sR35, and Rc_sR42) and known bacterial sRNAs (6S, RNaseP_bact_a, ffs, and α-tmRNA) was next confirmed by Northern hybridization. Comparative analysis during infection of HMECs vis-à-vis tick AAE2 cells revealed significantly higher expression of Rc_sR35 and Rc_sR42 in HMECs, whereas Rc_sR31 and Rc_sR33 were expressed at similar levels in both cell types. We further predicted a total of 502 genes involved in all important biological processes as potential targets of Rc_sRs and validated the interaction of Rc_sR42 with cydA (cytochrome d ubiquinol oxidase subunit I). Our findings constitute the first evidence of the existence of post-transcriptional riboregulatory mechanisms in R. conorii and interactions between a novel Rc_sR and its target mRNA.

Legionella pneumophila strain associated with the first evidence of person-to-person transmission of Legionnaires' disease: a unique mosaic genetic backbone

A first strong evidence of person-to-person transmission of Legionnaires’ Disease (LD) was recently reported. Here, we characterize the genetic backbone of this case-related Legionella pneumophila strain (“PtVFX/2014”), which also caused a large outbreak of LD. PtVFX/2014 is phylogenetically divergent from the most worldwide studied outbreak-associated L. pneumophila subspecies pneumophila serogroup 1 strains. In fact, this strain is also from serogroup 1, but belongs to the L. pneumophila subspecies fraseri. Its genomic mosaic backbone reveals eight horizontally transferred regions encompassing genes, for instance, involved in lipopolysaccharide biosynthesis or encoding virulence-associated Dot/Icm type IVB secretion system (T4BSS) substrates. PtVFX/2014 also inherited a rare ~65 kb pathogenicity island carrying virulence factors and detoxifying enzymes believed to contribute to the emergence of best-fitted strains in water reservoirs and in human macrophages, as well as a inter-species transferred (from L. oakridgensis) ~37.5 kb genomic island (harboring a lvh/lvr T4ASS cluster) that had never been found intact within L. pneumophila species. PtVFX/2014 encodes another lvh/lvr cluster near to CRISPR-associated genes, which may boost L. pneumophila transition from an environmental bacterium to a human pathogen. Overall, this unique genomic make-up may impact PtVFX/2014 ability to adapt to diverse environments, and, ultimately, to be transmitted and cause human disease.

Active and adaptive Legionella CRISPR-Cas reveals a recurrent challenge to the pathogen

Clustered regularly interspaced short palindromic repeats with CRISPR‐associated gene (CRISPR‐Cas) systems are widely recognized as critical genome defense systems that protect microbes from external threats such as bacteriophage infection. Several isolates of the intracellular pathogen Legionella pneumophila possess multiple CRISPR‐Cas systems (type I‐C, type I‐F and type II‐B), yet the targets of these systems remain unknown. With the recent observation that at least one of these systems (II‐B) plays a non‐canonical role in supporting intracellular replication, the possibility remained that these systems are vestigial genome defense systems co‐opted for other purposes. Our data indicate that this is not the case. Using an established plasmid transformation assay, we demonstrate that type I‐C, I‐F and II‐B CRISPR‐Cas provide protection against spacer targets. We observe efficient laboratory acquisition of new spacers under ‘priming’ conditions, in which initially incomplete target elimination leads to the generation of new spacers and ultimate loss of the invasive DNA. Critically, we identify the first known target of L. pneumophila CRISPR‐Cas: a 30 kb episome of unknown function whose interbacterial transfer is guarded against by CRISPR‐Cas. We provide evidence that the element can subvert CRISPR‐Cas by mutating its targeted sequences – but that primed spacer acquisition may limit this mechanism of escape. Rather than generally impinging on bacterial fitness, this element drives a host specialization event – with improved fitness in Acanthamoeba but a reduced ability to replicate in other hosts and conditions. These observations add to a growing body of evidence that host range restriction can serve as an existential threat to L. pneumophila in the wild.

Transcriptome-Wide Identification of Hfq-Associated RNAs in Brucella suis by Deep Sequencing

Recent breakthroughs in next-generation sequencing technologies have led to the identification of small noncoding RNAs (sRNAs) as a new important class of regulatory molecules. In prokaryotes, sRNAs are often bound to the chaperone protein Hfq, which allows them to interact with their partner mRNA(s). We screened the genome of the zoonotic and human pathogen Brucella suis 1330 for the presence of this class of RNAs. We designed a coimmunoprecipitation strategy that relies on the use of Hfq as a bait to enrich the sample with sRNAs and eventually their target mRNAs. By deep sequencing analysis of the Hfq-bound transcripts, we identified a number of mRNAs and 33 sRNA candidates associated with Hfq. The expression of 10 sRNAs in the early stationary growth phase was experimentally confirmed by Northern blotting and/or reverse transcriptase PCR.

IMPORTANCE

Brucella organisms are facultative intracellular pathogens that use stealth strategies to avoid host defenses. Adaptation to the host environment requires tight control of gene expression. Recently, small noncoding RNAs (sRNAs) and the sRNA chaperone Hfq have been shown to play a role in the fine-tuning of gene expression. Here we have used RNA sequencing to identify RNAs associated with the B. suis Hfq protein. We have identified a novel list of 33 sRNAs and 62 Hfq-associated mRNAs for future studies aiming to understand the intracellular lifestyle of this pathogen.

Phenotypic characterization and complementation analysis of Bacillus subtilis 6S RNA single and double deletion mutants

6S RNA, a global regulator of transcription in bacteria, binds to housekeeping RNA polymerase (RNAP) holoenzymes to competitively inhibit transcription from DNA promoters. Bacillus subtilis encodes two 6S RNA homologs whose differential functions are as yet unclear. We constructed derivative strains of B. subtilis PY79 lacking 6S-1 RNA (ΔbsrA), 6S-2 RNA (ΔbsrB) or both (ΔbsrAB) to study the physiological role of the two 6S RNAs. We observed two growth phenotypes of mutant strains: (i) accelerated decrease of optical density toward extended stationary phase and (ii) faster outgrowth from stationary phase under alkaline stress conditions (pH 9.8). The first phenotype was observed for bacteria lacking bsrA, and even more pronounced for ΔbsrAB bacteria, but not for those lacking bsrB. The magnitude of the second phenotype was relatively weak for ΔbsrB, moderate for ΔbsrA and again strongest for ΔbsrAB bacteria. Whereas ΔbsrAB bacteria complemented with bsrB or bsrA (strains ΔbsrAB + B and ΔbsrAB + A) mimicked the phenotypes of the ΔbsrA and ΔbsrB strains, respectively, complementation with the gene ssrS encoding Escherichia coli 6S RNA failed to cure the "low stationary optical density" phenotype of the double mutant, despite ssrS expression, in line with previous findings. Finally, proteomics (two-dimensional differential gel electrophoresis, 2D-DIGE) of B. subtilis 6S RNA deletion strains unveiled a set of proteins that were expressed at higher levels particularly during exponential growth and preferentially in mutant strains lacking 6S-2 RNA. Several of these proteins are involved in metabolism and stress responses.

Sibling rivalry: related bacterial small RNAs and their redundant and non-redundant roles

Small RNA molecules (sRNAs) are now recognized as key regulators controlling bacterial gene expression, as sRNAs provide a quick and efficient means of positively or negatively altering the expression of specific genes. To date, numerous sRNAs have been identified and characterized in a myriad of bacterial species, but more recently, a theme in bacterial sRNAs has emerged: the presence of more than one highly related sRNAs produced by a given bacterium, here termed sibling sRNAs. Sibling sRNAs are those that are highly similar at the nucleotide level, and while it might be expected that sibling sRNAs exert identical regulatory functions on the expression of target genes based on their high degree of relatedness, emerging evidence is demonstrating that this is not always the case. Indeed, there are several examples of bacterial sibling sRNAs with non-redundant regulatory functions, but there are also instances of apparent regulatory redundancy between sibling sRNAs. This review provides a comprehensive overview of the current knowledge of bacterial sibling sRNAs, and also discusses important questions about the significance and evolutionary implications of this emerging class of regulators.

6S RNA: recent answers--future questions

6S RNA is a non-coding RNA, found in almost all phylogenetic branches of bacteria. Through its conserved secondary structure, resembling open DNA promoters, it binds to RNA polymerase and interferes with transcription at many promoters. That way, it functions as transcriptional regulator facilitating adaptation to stationary phase conditions. Strikingly, 6S RNA acts as template for the synthesis of small RNAs (pRNA), which trigger the disintegration of the inhibitory RNA polymerase-6S RNA complex releasing 6S RNA-dependent repression. The regulatory implications of 6S RNAs vary among different bacterial species depending on the lifestyle and specific growth conditions that they have to face. The influence of 6S RNA can be seen on many different processes including stationary growth, sporulation, light adaptation or intracellular growth of pathogenic bacteria. Recent structural and functional studies have yielded details of the interaction between E. coli 6S RNA and RNA polymerase. Genome-wide transcriptome analyses provided insight into the functional diversity of 6S RNAs. Moreover, the mechanism and physiological consequences of pRNA synthesis have been explored in several systems. A major function of 6S RNA as a guardian regulating the economic use of cellular resources under limiting conditions and stress emerges as a common perception from numerous recent studies.

Analysis of the transcriptome of Legionella pneumophila hfq mutant reveals a new mobile genetic element

Hfq is a small RNA-binding protein involved in the post-transcriptional regulation of gene expression by affecting the stability of the mRNA and by mediating efficient pairing between small regulatory RNAs and their target mRNAs. In Legionella pneumophila, the aetiological agent of Legionnaires' disease, mutation of hfq results in increased duration of the lag phase and reduced growth in low-iron medium. In an effort to uncover genes potentially regulated by Hfq, the transcriptome of an hfq mutant strain was compared to that of the wild-type. Unexpectedly, many genes located within a 100 kb genomic island, including a section of the previously identified efflux island, were overexpressed in the hfq mutant strain. Since this island contains a putative conjugative system and an integrase, it was postulated that it could be a new integrated mobile genetic element. PCR analysis revealed that this region exists both as an integrated and as an episomal form in the cell population and that it undergoes differential excision in the hfq mutant background, which was further confirmed by trans-complementation of the hfq mutation. This new plasmid-like element was named pLP100. Differential excision did not affect the copy number of pLP100 at the population level. This region contains a copper efflux pump encoded by copA, and increased resistance to copper was observed for the hfq mutant strain that was abrogated in the complemented strain. A strain carrying a mutation of hfq and a deletion of the right side recombination site, attR, showed that overexpression of pLP100 genes and increased copper resistance in the hfq mutant strain were dependent upon excision of pLP100.

The CRISPR-associated gene cas2 of Legionella pneumophila is required for intracellular infection of amoebae

Recent studies have shown that the clustered regularly interspaced palindromic repeats (CRISPR) array and its associated (cas) genes can play a key role in bacterial immunity against phage and plasmids. Upon analysis of the Legionella pneumophila strain 130b chromosome, we detected a subtype II-B CRISPR-Cas locus that contains cas9, cas1, cas2, cas4, and an array with 60 repeats and 58 unique spacers. Reverse transcription (RT)-PCR analysis demonstrated that the entire CRISPR-Cas locus is expressed during 130b extracellular growth in both rich and minimal media as well as during intracellular infection of macrophages and aquatic amoebae. Quantitative reverse transcription-PCR (RT-PCR) further showed that the levels of cas transcripts, especially those of cas1 and cas2, are elevated during intracellular growth relative to exponential-phase growth in broth. Mutants lacking components of the CRISPR-Cas locus were made and found to grow normally in broth and on agar media. cas9, cas1, cas4, and CRISPR array mutants also grew normally in macrophages and amoebae. However, cas2 mutants, although they grew typically in macrophages, were significantly impaired for infection of both Hartmannella and Acanthamoeba species. A complemented cas2 mutant infected the amoebae at wild-type levels, confirming that cas2 is required for intracellular infection of these host cells.

IMPORTANCE

Given that infection of amoebae is critical for L. pneumophila persistence in water systems, our data indicate that cas2 has a role in the transmission of Legionnaires' disease. Because our experiments were done in the absence of added phage, plasmid, or nucleic acid, the event that is facilitated by Cas2 is uniquely distinct from current dogma concerning CRISPR-Cas function.

Facets of small RNA-mediated regulation in Legionella pneumophila

Legionella pneumophila is a water-borne pathogen that causes a severe lung infection in humans. It is able to replicate inside amoeba in the water environment, and inside lung macrophages in humans. Efficient regulation of gene expression is critical for responding to the conditions that L. pneumophila encounters and for intracellular multiplication in host cells. In the last two decades, many reports have contributed to our understanding of the critical importance of small regulatory RNAs (sRNAs) in the regulatory network of bacterial species. This report presents the current state of knowledge about the sRNAs expressed by L. pneumophila and discusses a few regulatory pathways in which sRNAs should be involved in this pathogen.

Virulence phenotypes of Legionella pneumophila associated with noncoding RNA lpr0035

The Philadelphia-1 strain of Legionella pneumophila, the causative organism of Legionnaires' disease, contains a recently discovered noncoding RNA, lpr0035. lpr0035 straddles the 5' chromosomal junction of a 45-kbp mobile genetic element, pLP45, which can exist as an episome or integrated in the bacterial chromosome. A 121-bp deletion was introduced in strain JR32, a Philadelphia-1 derivative. The deletion inactivated lpr0035, removed the 49-bp direct repeat at the 5' junction of pLP45, and locked pLP45 in the chromosome. Intracellular multiplication of the deletion mutant was decreased by nearly 3 orders of magnitude in Acanthamoeba castellanii amoebae and nearly 2 orders of magnitude in J774 mouse macrophages. Entry of the deletion mutant into amoebae and macrophages was decreased by >70%. The level of entry in both hosts was restored to that in strain JR32 by plasmid copies of two open reading frames immediately downstream of the 5' junction and plasmid lpr0035 driven by its endogenous promoter. When induced from a tac promoter, plasmid lpr0035 completely reversed the intracellular multiplication defect in macrophages but was without effect in amoebae. These data are the first evidence of a role for noncoding RNA lpr0035, which has homologs in six other Legionella genomes, in entry of L. pneumophila into amoebae and macrophages and in host-specific intracellular multiplication. The data also demonstrate that deletion of a direct-repeat sequence restricts the mobility of pLP45 and is a means of studying the role of pLP45 mobility in Legionella virulence phenotypes.