Genetic manipulation of rickettsiae: a preview

Subversion of host cell signaling: The arsenal of Rickettsial species

Rickettsia is a genus of nonmotile, Gram-negative, non-spore-forming, highly pleomorphic bacteria that cause severe epidemic rickettsioses. The spotted fever group and typhi group are major members of the genus Rickettsia. Rickettsial species from the two groups subvert diverse host cellular processes, including membrane dynamics, actin cytoskeleton dynamics, phosphoinositide metabolism, intracellular trafficking, and immune defense, to promote their host colonization and intercellular transmission through secreted effectors (virulence factors). However, lineage-specific rickettsiae have exploited divergent strategies to accomplish such challenging tasks and these elaborated strategies focus on distinct host cell processes. In the present review, we summarized current understandings of how different rickettsial species employ their effectors' arsenal to affect host cellular processes in order to promote their own replication or to avoid destruction.

Rickettsia typhi possesses phospholipase A2 enzymes that are involved in infection of host cells

The long-standing proposal that phospholipase A2 (PLA2) enzymes are involved in rickettsial infection of host cells has been given support by the recent characterization of a patatin phospholipase (Pat2) with PLA2 activity from the pathogens Rickettsia prowazekii and R. typhi. However, pat2 is not encoded in all Rickettsia genomes; yet another uncharacterized patatin (Pat1) is indeed ubiquitous. Here, evolutionary analysis of both patatins across 46 Rickettsia genomes revealed 1) pat1 and pat2 loci are syntenic across all genomes, 2) both Pat1 and Pat2 do not contain predicted Sec-dependent signal sequences, 3) pat2 has been pseudogenized multiple times in rickettsial evolution, and 4) ubiquitous pat1 forms two divergent groups (pat1A and pat1B) with strong evidence for recombination between pat1B and plasmid-encoded homologs. In light of these findings, we extended the characterization of R. typhi Pat1 and Pat2 proteins and determined their role in the infection process. As previously demonstrated for Pat2, we determined that 1) Pat1 is expressed and secreted into the host cytoplasm during R. typhi infection, 2) expression of recombinant Pat1 is cytotoxic to yeast cells, 3) recombinant Pat1 possesses PLA2 activity that requires a host cofactor, and 4) both Pat1 cytotoxicity and PLA2 activity were reduced by PLA2 inhibitors and abolished by site-directed mutagenesis of catalytic Ser/Asp residues. To ascertain the role of Pat1 and Pat2 in R. typhi infection, antibodies to both proteins were used to pretreat rickettsiae. Subsequent invasion and plaque assays both indicated a significant decrease in R. typhi infection compared to that by pre-immune IgG. Furthermore, antibody-pretreatment of R. typhi blocked/delayed phagosomal escapes. Together, these data suggest both enzymes are involved early in the infection process. Collectively, our study suggests that R. typhi utilizes two evolutionary divergent patatin phospholipases to support its intracellular life cycle, a mechanism distinguishing it from other rickettsial species.

Development of shuttle vectors for transformation of diverse Rickettsia species

Plasmids have been identified in most species of Rickettsia examined, with some species maintaining multiple different plasmids. Three distinct plasmids were demonstrated in Rickettsia amblyommii AaR/SC by Southern analysis using plasmid specific probes. Copy numbers of pRAM18, pRAM23 and pRAM32 per chromosome in AaR/SC were estimated by real-time PCR to be 2.0, 1.9 and 1.3 respectively. Cloning and sequencing of R. amblyommii AaR/SC plasmids provided an opportunity to develop shuttle vectors for transformation of rickettsiae. A selection cassette encoding rifampin resistance and a fluorescent marker was inserted into pRAM18 yielding a 27.6 kbp recombinant plasmid, pRAM18/Rif/GFPuv. Electroporation of Rickettsia parkeri and Rickettsia bellii with pRAM18/Rif/GFPuv yielded GFPuv-expressing rickettsiae within 2 weeks. Smaller vectors, pRAM18dRG, pRAM18dRGA and pRAM32dRGA each bearing the same selection cassette, were made by moving the parA and dnaA-like genes from pRAM18 or pRAM32 into a vector backbone. R. bellii maintained the highest numbers of pRAM18dRGA (13.3 – 28.1 copies), and R. parkeri, Rickettsia monacensis and Rickettsia montanensis contained 9.9, 5.5 and 7.5 copies respectively. The same species transformed with pRAM32dRGA maintained 2.6, 2.5, 3.2 and 3.6 copies. pRM, the plasmid native to R. monacensis, was still present in shuttle vector transformed R. monacensis at a level similar to that found in wild type R. monacensis after 15 subcultures. Stable transformation of diverse rickettsiae was achieved with a shuttle vector system based on R. amblyommii plasmids pRAM18 and pRAM32, providing a new research tool that will greatly facilitate genetic and biological studies of rickettsiae.

Advances in genetic manipulation of obligate intracellular bacterial pathogens

Infections by obligate intracellular bacterial pathogens result in significant morbidity and mortality worldwide. These bacteria include Chlamydia spp., which causes millions of cases of sexually transmitted disease and blinding trachoma annually, and members of the α-proteobacterial genera Anaplasma, Ehrlichia, Orientia, and Rickettsia, agents of serious human illnesses including epidemic typhus. Coxiella burnetii, the agent of human Q fever, has also been considered a prototypical obligate intracellular bacterium, but recent host cell-free (axenic) growth has rescued it from obligatism. The historic genetic intractability of obligate intracellular bacteria has severely limited molecular dissection of their unique lifestyles and virulence factors involved in pathogenesis. Host cell restricted growth is a significant barrier to genetic transformation that can make simple procedures for free-living bacteria, such as cloning, exceedingly difficult. Low transformation efficiency requiring long-term culture in host cells to expand small transformant populations is another obstacle. Despite numerous technical limitations, the last decade has witnessed significant gains in genetic manipulation of obligate intracellular bacteria including allelic exchange. Continued development of genetic tools should soon enable routine mutation and complementation strategies for virulence factor discovery and stimulate renewed interest in these refractory pathogens. In this review, we discuss the technical challenges associated with genetic transformation of obligate intracellular bacteria and highlight advances made with individual genera.

Review of microarray studies for host-intracellular pathogen interactions

Obligate intracellular bacteria are privileged soldiers on the battlefield that represent host-pathogen interactions. Microarrays are a powerful technology that can increase our knowledge about how bacteria respond to and interact with their hosts. This review summarizes the limitations inherent to host-pathogen interaction studies and essential strategies to improve microarray investigations of intracellular bacteria. We have compiled the comparative genomic and gene expression analyses of obligate intracellular bacteria currently available from microarrays. In this review we explore ways in which microarrays can be used to identify polymorphisms in different obligate intracellular bacteria such as Coxiella burnetii, Chlamydia trachomatis, Ehrlichia chaffeensis, Rickettsia prowazekii and Tropheryma whipplei. These microarray studies reveal that, while genomic content is highly conserved in obligate intracellular bacteria, genetic polymorphisms can potentially occur to increase bacterial pathogenesis. Additionally, changes in the gene expression of C. trachomatis throughout its life cycle, as well as changes in the gene expression profile of the pathogens R. prowazekii, Rickettsia rickettsii, Rickettsia typhi, T. whipplei and C. trachomatis following environmental changes, are discussed. Finally, an in vivo model of Rickettsia conorii within the skin is discussed. The gene expression analyses highlight the capacity of obligate intracellular bacteria to adapt to environmental changes and potentially to thwart the host response.

Progress in the functional analysis of rickettsial genes through directed mutagenesis of Rickettsia prowazekii phospholipase D

Evaluation of: Driskell LO, Yu X-J, Zhang L et al.: Directed mutagenesis of the Rickettsia prowazekii pld gene encoding phospholipase D. Infect. Immun. 77(8), 3244–3248 (2009). Rickettsioses have afflicted humans worldwide throughout the course of history. Rickettsia prowazekii is the etiological agent of epidemic typhus, a disease transmitted by body lice and capable of massive outbreaks under conditions of compromised hygiene, such as famine, mass migration and war. Fastidious growth requirements and an obligately intracellular lifestyle, preferably within the cytoplasm of the host cell, pose a number of challenges in genetic manipulation of rickettsiae. Driskell et al. describe the successful application of a combination of molecular approaches to generate, isolate and characterize a R. prowazekii mutant lacking a 93-bp sequence of the pld gene responsible for phospholipase D activity. In initial studies, this Δpld mutant of R. prowazekii is shown to be capable of infecting macrophage-like RAW 264.7 cells in culture and quickly escaping from the phagosome into the cytosol. However, in guinea pigs infected by intraperitoneal inoculation, the pld deletion mutant exhibits attenuation of virulence and the ability to induce protective immune responses against virulent R. prowazekii. The fundamental importance of this study lies in the generation of a site-directed gene mutant for subsequent evaluation of the target gene’s function(s) in rickettsial pathogenesis and immune defense mechanisms. The results also lend support to the possibility that rickettsiae may either possess multiple phosholipases with different substrate specificities or as yet unknown alternative mechanisms for quick phagosomal escape into the host cytoplasm. Studies aimed at detailed characterization of the combinatorial mutant generated in this study and possibly other complete knockouts for genes with putative functions using relevant in vitro and in vivo models of infection are necessary to further elucidate their roles in the biology of rickettsiae.

Rickettsial ompB promoter regulated expression of GFPuv in transformed Rickettsia montanensis

Background

Rickettsia spp. (Rickettsiales: Rickettsiaceae) are Gram-negative, obligate intracellular, α-proteobacteria that have historically been associated with blood-feeding arthropods. Certain species cause typhus and spotted fevers in humans, but others are of uncertain pathogenicity or may be strict arthropod endosymbionts. Genetic manipulation of rickettsiae should facilitate a better understanding of their interactions with hosts.

Methodology/Principal Findings

We transformed a species never associated with human disease, Rickettsia montanensis, by electroporation with a TN5 transposon (pMOD700) containing green fluorescent protein (GFPuv) and chloramphenicol acetyltransferase (CAT) genes under regulation of promoters cloned from the Rickettsia rickettsii ompB gene, and isolated a Chloramphenicol-resistant GFP-fluorescent rickettsiae population (Rmontanensis700). The Rmontanensis700 rickettsiae contained a single transposon integrated near an acetyl-CoA acetyltransferase gene in the rickettsial chromosome. Northern blots showed that GFPuv and CAT mRNAs were both expressed as two transcripts of larger and smaller than predicted length. Western immunoblots showed that Rmontanensis700 and E. coli transformed with a plasmid containing the pMOD700 transposon both expressed GFPuv proteins of the predicted molecular weight.

Conclusions/Significance

Long-standing barriers to transformation of rickettsiae have been overcome by development of transposon-based rickettsial transformation vectors. The ompB promoter may be the most problematic of the four promoters so far employed in those vectors.

Advances in rickettsia pathogenicity

One century after the first description of rickettsiae as human pathogens, the rickettsiosis remained poorly understood diseases. These microorganisms are indeed characterized by a strictly intracellular location which has, for long, prohibited their detailed study. Within the last ten years, the completion of the genome sequences of several strains allowed gaining a better knowledge about the molecular mechanisms involved in rickettsia pathogenicity. Here, we summarized available data concerning the critical steps of rickettsia-host cell interactions that should contribute to tissue injury and diseases, that is, adhesion, phagosomal escape, motility, and intracellular survival of the bacteria.

Regulation of whole bacterial pathogen transcription within infected hosts

DNA microarrays are a powerful and promising approach to gain a detailed understanding of the bacterial response and the molecular cross-talk that can occur as a consequence of host-pathogen interactions. However, published studies mainly describe the host response to infection. Analysis of bacterial gene regulation in the course of infection has confronted many challenges. This review summarizes the different strategies used over the last few years to investigate, at the genomic scale, and using microarrays, the alterations in the bacterial transcriptome in response to interactions with host cells. Thirty-seven studies involving 19 different bacterial pathogens were compiled and analyzed. Our in silico comparison of the transcription profiles of bacteria grown in broth or in contact with eukaryotic cells revealed some features commonly observed when bacteria interact with host cells, including stringent response and cell surface remodeling.

Development of a method for recovering rickettsial RNA from infected cells to analyze gene expression profiling of obligate intracellular bacteria

The Rickettsia genus is composed of Gram-negative bacteria responsible for Typhus and spotted fevers. Because of the limitations imposed by their obligate intracellular location, the molecular mechanisms responsible for their pathogenicity remain poorly understood. Several rickettsial genomes are now available, thus providing the foundation for a new era of post-genomic research. Here, using Rickettsia conorii as model, we developed a suitable method for microarray-based transcriptome analysis of rickettsiae. Total RNA was extracted from infected Vero cells using a protocol preserving its integrity, as observed by Bioanalyzer (Agilent) profiles. By a subtractive hybridization method, the samples were subsequently depleted of eukaryotic RNA that represents up to 90% of the whole extract and that hampers fluorochrome labeling of rickettsial nucleic acids. To obtain the amount of material required for microarray hybridization, the bacterial RNA was then amplified using random primers. Hybridizations were carried out on microarrays specific for R. conorii but containing a limited number of selected targets. Our results show that this method yielded reproducible signals. Transcriptional changes observed following exposure of R. conorii to a nutrient stress were verified by real-time quantitative PCR and by quantitative reverse transcription PCR starting from amplified cDNA and total RNA as templates, respectively. We conclude that this approach has great potential for the study of mechanisms behind the virulence and intracellular survival of members of the genus Rickettsia.

Transposon insertion reveals pRM, a plasmid of Rickettsia monacensis

Until the recent discovery of pRF in Rickettsia felis, the obligate intracellular bacteria of the genus Rickettsia (Rickettsiales: Rickettsiaceae) were thought not to possess plasmids. We describe pRM, a plasmid from Rickettsia monacensis, which was detected by pulsed-field gel electrophoresis and Southern blot analyses of DNA from two independent R. monacensis populations transformed by transposon-mediated insertion of coupled green fluorescent protein and chloramphenicol acetyltransferase marker genes into pRM. Two-dimensional electrophoresis showed that pRM was present in rickettsial cells as circular and linear isomers. The 23,486-nucleotide (31.8% G/C) pRM plasmid was cloned from the transformant populations by chloramphenicol marker rescue of restriction enzyme-digested transformant DNA fragments and PCR using primers derived from sequences of overlapping restriction fragments. The plasmid was sequenced. Based on BLAST searches of the GenBank database, pRM contained 23 predicted genes or pseudogenes and was remarkably similar to the larger pRF plasmid. Two of the 23 genes were unique to pRM and pRF among sequenced rickettsial genomes, and 4 of the genes shared by pRM and pRF were otherwise found only on chromosomes of R. felis or the ancestral group rickettsiae R. bellii and R. canadensis. We obtained pulsed-field gel electrophoresis and Southern blot evidence for a plasmid in R. amblyommii isolate WB-8-2 that contained genes conserved between pRM and pRF. The pRM plasmid may provide a basis for the development of a rickettsial transformation vector.

The lspA gene, encoding the type II signal peptidase of Rickettsia typhi: transcriptional and functional analysis

Lipoprotein processing by the type II signal peptidase (SPase II) is known to be critical for intracellular growth and virulence for many bacteria, but its role in rickettsiae is unknown. Here, we describe the analysis of lspA, encoding a putative SPase II, an essential component of lipoprotein processing in gram-negative bacteria, from Rickettsia typhi. Alignment of deduced amino acid sequences shows the presence of highly conserved residues and domains that are essential for SPase II activity in lipoprotein processing. The transcription of lspA, lgt (encoding prolipoprotein transferase), and lepB (encoding type I signal peptidase), monitored by real-time quantitative reverse transcription-PCR, reveals a differential expression pattern during various stages of rickettsial intracellular growth. The higher transcriptional level of all three genes at the preinfection time point indicates that only live and metabolically active rickettsiae are capable of infection and inducing host cell phagocytosis. lspA and lgt, which are involved in lipoprotein processing, show similar levels of expression. However, lepB, which is involved in nonlipoprotein secretion, shows a higher level of expression, suggesting that LepB is the major signal peptidase for protein secretion and supporting our in silico prediction that out of 89 secretory proteins, only 14 are lipoproteins. Overexpression of R. typhi lspA in Escherichia coli confers increased globomycin resistance, indicating its function as SPase II. In genetic complementation, recombinant lspA from R. typhi significantly restores the growth of temperature-sensitive E. coli Y815 at the nonpermissive temperature, supporting its biological activity as SPase II in prolipoprotein processing.

Actin-based motility of intracellular bacteria, and polarized surface distribution of the bacterial effector molecules

Several intracellular bacterial pathogens, including species of Listeria, Rickettsia, Shigella, Mycobacteria, and Burkholderia, have evolved mechanisms to exploit the actin polymerization machinery of their hosts to induce actin-based motility, enabling these pathogens to spread between host cells without exposing themselves to the extracellular milieu. Efficient cell-to-cell spread requires directional motility, which the bacteria may achieve by concentrating the effector molecules at one pole of their cell body, thereby restricting polymerization of monomeric actin into actin tails to this pole. The study of the molecular processes involved in the initiation of actin tail formation at the bacterial surface, and subsequent actin-based motility, has provided much insight into the pathogenesis of infections caused by these bacteria and into the cell biology of actin dynamics. Concomitantly, this field of research has provided an opportunity to understand the mechanisms whereby bacteria can achieve a polarized distribution of surface proteins. This review will describe the process of actin-based motility of intracellular bacteria, and the mechanisms by which bacteria can obtain a polarized distribution of their surface proteins.

A novel and naturally occurring transposon, ISRpe1 in the Rickettsia peacockii genome disrupting the rickA gene involved in actin-based motility

While examining the molecular basis for the lack of actin-based motility for the non-pathogenic spotted fever group (SFG) R. peacockii, we identified a novel insertion sequence (IS) element, ISRpe1, disrupting the coding sequence of rickA, demonstrated to induce actin-tail polymerization for the SFG rickettsiae. This rickettsial IS element appears to be active in that complete terminal inverted repeat and recombinase/transposase open reading frame sequences are present and the transposase is transcriptionally expressed. Phylogenetically, ISRpe1 belongs to a new IS family that is most closely related to those transposable elements of other intracellular bacteria like Wolbachia spp. ISRpe1 was demonstrated to be present in at least 10 locations throughout the R. peacockii genome, including one that disrupted the putative cell surface antigen encoding gene, sca1 considered to be involved in adhesion and virulence of the rickettsiae. Additionally, three IS sites demonstrated rearrangements/relocations of the R. peacockii genome when compared to those of other SFG rickettsiae. Our findings of the disruptions of rickA and sca1 along with the comparative genomic reassortments associated with ISRpe1 in the non-virulent R. peacockii provides opportunities to uncover molecular mechanisms underlying the pathogenesis and evolution of rickettsiae as well as its potential to be used in rickettsial transposon-based mutagenesis.

Identification of two putative rickettsial adhesins by proteomic analysis

The rickettsial membrane proteins that promote their uptake by eukaryotic host cells are unknown. To identify rickettsial ligand(s) that bind host cell surface proteins, biotinylated epithelial cells were used to probe a nitrocellulose membrane containing rickettsial extracts separated by SDS-PAGE. This overlay assay revealed that two close rickettsial ligands of approximately 32-30 kDa were recognized by host cells. Both proteins were identified using high resolution 2D-PAGE coupled with mass spectrometry analysis. One protein was identified as the C-terminal extremity of rOmpB called the beta-peptide. The second interacting protein was identified as a protein of unknown function encoded by RC1281 and RP828 in Rickettsia conorii and in Rickettsia prowazekii, respectively, that shares strong similarities with other bacterial adhesins. Both proteins are highly conserved within the Rickettsia genus and might play a critical role in their pathogenicity. These data may have important implications for the development of future vaccines against rickettsial infections.

Analysis of fluorescent protein expression in transformants of Rickettsia monacensis, an obligate intracellular tick symbiont

We developed and applied transposon-based transformation vectors for molecular manipulation and analysis of spotted fever group rickettsiae, which are obligate intracellular bacteria that infect ticks and, in some cases, mammals. Using the Epicentre EZ::TN transposon system, we designed transposons for simultaneous expression of a reporter gene and a chloramphenicol acetyltransferase (CAT) resistance marker. Transposomes (transposon-transposase complexes) were electroporated into Rickettsia monacensis, a rickettsial symbiont isolated from the tick Ixodes ricinus. Each transposon contained an expression cassette consisting of the rickettsial ompA promoter and a green fluorescent protein (GFP) reporter gene (GFPuv) or the ompB promoter and a red fluorescent protein reporter gene (DsRed2), followed by the ompA transcription terminator and a second ompA promoter CAT gene cassette. Selection with chloramphenicol gave rise to rickettsial populations with chromosomally integrated single-copy transposons as determined by PCR, Southern blotting, and sequence analysis. Reverse transcription-PCR and Northern blots demonstrated transcription of all three genes. GFPuv transformant rickettsiae exhibited strong fluorescence in individual cells, but DsRed2 transformants did not. Western blots confirmed expression of GFPuv in R. monacensis and in Escherichia coli, but DsRed2 was expressed only in E. coli. The DsRed2 gene, but not the GFPuv gene, contains many GC-rich amino acid codons that are rare in the preferred codon suite of rickettsiae, possibly explaining the failure to express DsRed2 protein in R. monacensis. We demonstrated that our vectors provide a means to study rickettsia-host cell interactions by visualizing GFPuv-fluorescent R. monacensis associated with actin tails in tick host cells.

Some lessons from Rickettsia genomics

Sequencing of the Rickettsia conorii genome and its comparison with its closest sequenced pathogenic relative, i.e., Rickettsia prowazekii, provided powerful insights into the evolution of these microbial pathogens. However, advances in our knowledge of rickettsial diseases are still hindered by the difficulty of working with strict intracellular bacteria and their hosts. Information gained from comparing the genomes of closely related organisms will shed new light on proteins susceptible to be targeted in specific diagnostic assays, by new antimicrobial drugs, and that could be employed in the generation of future rickettsial vaccines. In this review we present a detailed comparison of the metabolic pathways of these bacteria as well as the polymorphisms of their membrane proteins, transporters and putative virulence factors. Environmental adaptation of Rickettsia is also discussed.

Transcriptional response of Rickettsia conorii exposed to temperature variation and stress starvation

Rickettsia conorii is an obligate intracellular bacterium transmitted to humans by Rhipicephalus sanguineus ticks. The success of this microorganism at surviving in nature implicates the ability to efficiently adapt to different environments, including the arthropod vector and the mammalian host. Numerous bacterial species possess a highly evolved system for stress adaptation. This so-called stringent response is mediated by guanosine 3',5'-bispyrophosphate and guanosine 3'-diphosphate 5'-triphosphate which are under spoT control in some Gram-negative bacteria. Interestingly, annotation of the R. conorii genome evidenced 5 spoT paralogs. We hypothesized that these spoT genes play a role in adaptation to environmental changes specifically encountered by rickettsiae during their different life cycles. Transcription of the spoT paralogs was examined by RT-PCR from infected Vero cells maintained in rich or deficient culture media, from infected C6/36 insect cells cultured at various temperatures and from infected ticks. Our results demonstrated that the 5 spoT genes can be transcribed. SpoT1 (RC0374) is only transcribed upon stringent response. Transcription of spoT3 (RC0888) was never observed in arthropod cells or ticks, but was specific to R. conorii RNA extracted from infected Vero cells. These results indicate that rickettsial spoT paralogs are independently transcribed, depending on the different infected hosts and the adaptive capacity of the pathogen. Bioinformatics analysis of these possibly encoded proteins is also reported.

Molecular and functional analysis of the lepB gene, encoding a type I signal peptidase from Rickettsia rickettsii and Rickettsia typhi

The type I signal peptidase lepB genes from Rickettsia rickettsii and Rickettsia typhi, the etiologic agents of Rocky Mountain spotted fever and murine typhus, respectively, were cloned and characterized. Sequence analysis of the cloned lepB genes from R. rickettsii and R. typhi shows open reading frames of 801 and 795 nucleotides, respectively. Alignment analysis of the deduced amino acid sequences reveals the presence of highly conserved motifs that are important for the catalytic activity of bacterial type I signal peptidase. Reverse transcription-PCR and Northern blot analysis demonstrated that the lepB gene of R. rickettsii is cotranscribed in a polycistronic message with the putative nuoF (encoding NADH dehydrogenase I chain F), secF (encoding protein export membrane protein), and rnc (encoding RNase III) genes in a secF-nuoF-lepB-rnc cluster. The cloned lepB genes from R. rickettsii and R. typhi have been demonstrated to possess signal peptidase I activity in Escherichia coli preprotein processing in vivo by complementation assay.

From genes to proteins: in vitro expression of rickettsial proteins

The availability of the complete genome sequences of several organisms allows the comparative analysis of genomes, a branch of bioinformatics known as genomics. With this approach, much can be learned about the biology of organisms that are difficult to culture, even when few, if any, of their proteins have been isolated and studied directly. We have focused our interest on Rickettsia conorii, an obligate intracellular bacterium responsible for Mediterranean spotted fever, a disease endemic in southern Europe. While bioinformatic annotation of the complete genome of this bacteria has allowed identification of 1,374 genes, a large number of them remain functionally uncharacterized. The final goal of many experiments in molecular biology is to use biological systems to synthesize the protein encoded by the gene being studied. Because three-dimensional structures are more resilient to evolution and change than amino acid sequences, structure determination of some open reading frames should also exhibit structural similarity to previously described protein families. We have thus initiated a systematic expression and structure determination program for the proteins encoded by rickettsial genes of interest. We have cloned different genes of R. conorii by recombinational cloning (GATEWAY), Invitrogen) a method that uses in vitro site-specific recombination to accomplish a directional cloning of PCR products and the subsequent automatic subcloning of the DNA segment into new vector backbones at high efficiency. The constructions in p-Dest17 yielded several clones able to express recombinant proteins with a C-terminal histidine tag. Expression of corresponding proteins was then performed using a cell-free protein expression system (Rapid Translation System, RTS, Roche Diagnostics). The recombinational cloning approach coupled to RTS provides an approach to rapid optimization of protein expression and is very useful to express rickettsial proteins. Moreover, this system is able to overcome some of the limitations encountered with rickettsial proteins highly toxic for E. coli or insect cells.

Expression of green fluorescent protein in Rickettsia conorii

Rickettsiae are obligate intracellular class III pathogens for which genetic manipulation has only recently been shown to be feasible. Such experiments were restricted to the typhus group rickettsiae, namely R. typhi and R. prowazekii. Here we report the first genetic manipulation of Rickettsia conorii, the bacterial agent responsible for the Mediterranean spotted fever. A gene encoding a variant of the green fluorescent protein under the control of the sterically repressed promoter (srp) from E. coli was integrated into the genome of this bacteria and detected by FACS analysis.