Linda D. Hicks

Proteomic and immunoblot analyses of Bartonella quintana total membrane proteins identify antigens recognized by sera from infected patients

ABSTRACT Bartonella quintana is a fastidious, gram-negative, rod-shaped bacterium that causes prolonged bacteremia in immunocompetent humans and severe infections in immunocompromised individuals. We sought to define the outer membrane subproteome of B. quintana in order to obtain insight into the biology and pathogenesis of this emerging pathogen and to identify the predominant B. quintana antigens targeted by the human immune system during infection. We isolated the total membrane proteins of B. quintana and identified 60 proteins by two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis and peptide mass fingerprinting. Using the newly constructed proteome map, we then utilized two-dimensional immunoblotting with sera from 21 B. quintana-infected patients to identify 24 consistently recognized, immunoreactive B. quintana antigens that have potential relevance for pathogenesis and diagnosis. Among the outer membrane proteins, the variably expressed outer membrane protein adhesins (VompA and VompB), peptidyl-prolyl cis-trans-isomerase (PpI), and hemin-binding protein E (HbpE) were recognized most frequently by sera from patients, which is consistent with surface expression of these virulence factors during human infection.

Toxic Introns and Parasitic Intein in Coxiella burnetii: Legacies of a Promiscuous Past

The genome of the obligate intracellular pathogen Coxiella burnetii contains a large number of selfish genetic elements, including two group I introns (Cbu.L1917 and Cbu.L1951) and an intervening sequence that interrupts the 23S rRNA gene, an intein (Cbu.DnaB) within dnaB and 29 insertion sequences. Here, we describe the ability of the intron-encoded RNAs (ribozymes) to retard bacterial growth rate (toxicity) and examine the functionality and phylogenetic history of Cbu.DnaB. When expressed in Escherichia coli, both introns repressed growth, with Cbu.L1917 being more inhibitory. Both ribozymes were found to associate with ribosomes of Coxiella and E. coli. In addition, ribozymes significantly reduced in vitro luciferase translation, again with Cbu.L1917 being more inhibitory. We analyzed the relative quantities of ribozymes and genomes throughout a 14-day growth cycle of C. burnetii and found that they were inversely correlated, suggesting that the ribozymes have a negative effect on Coxiellas growth. We determined possible sites for ribozyme associations with 23S rRNA that could explain the observed toxicities. Further research is needed to determine whether the introns are being positively selected because they promote bacterial persistence or whether they were fixed in the population due to genetic drift. The intein, Cbu.DnaB, is able to self-splice, leaving the host protein intact and presumably functional. Similar inteins have been found in two extremophilic bacteria (Alkalilimnicola ehrlichei and Halorhodospira halophila) that are distantly related to Coxiella, making it difficult to determine whether the intein was acquired by horizontal gene transfer or was vertically inherited from a common ancestor.

The Unusual 23S rRNA Gene of Coxiella burnetii: Two Self-Splicing Group I Introns Flank a 34-Base-Pair Exon, and One Element Lacks the Canonical ΩG

We describe the presence and characteristics of two self-splicing group I introns in the sole 23S rRNA gene of Coxiella burnetii. The two group I introns, Cbu.L1917 and Cbu.L1951, are inserted at sites 1917 and 1951 (Escherichia coli numbering), respectively, in the 23S rRNA gene of C. burnetii. Both introns were found to be self-splicing in vivo and in vitro even though the terminal nucleotide of Cbu.L1917 is adenine and not the canonical conserved guanine, termed OmegaG, found in Cbu.L1951 and all other group I introns described to date. Predicted secondary structures for both introns were constructed and revealed that Cbu.L1917 and Cbu.L1951 were group IB2 and group IA3 introns, respectively. We analyzed strains belonging to eight genomic groups of C. burnetii to determine sequence variation and the presence or absence of the elements and found both introns to be highly conserved (>/=99%) among them. Although phylogenetic analysis did not identify the specific identities of donors, it indicates that the introns were likely acquired independently; Cbu.L1917 was acquired from other bacteria like Thermotoga subterranea and Cbu.L1951 from lower eukaryotes like Acanthamoeba castellanii. We also confirmed the fragmented nature of mature 23S rRNA in C. burnetii due to the presence of an intervening sequence. The presence of three selfish elements in C. burnetiis 23S rRNA gene is very unusual for an obligate intracellular bacterium and suggests a recent shift to its current lifestyle from a previous niche with greater opportunities for lateral gene transfer.

A DNA-Binding Peroxiredoxin of Coxiella burnetii Is Involved in Countering Oxidative Stress during Exponential-Phase Growth

Coxiella burnetii is a Gram-negative, obligate intracellular bacterial pathogen that resides within the harsh, acidic confines of a lysosome-like compartment of the host cell that is termed a parasitophorous vacuole. In this study, we characterized a thiol-specific peroxidase of C. burnetii that belongs to the atypical 2-cysteine subfamily of peroxiredoxins, commonly referred to as bacterioferritin comigratory proteins (BCPs). Coxiella BCP was initially identified as a potential DNA-binding protein by two-dimensional Southwestern (SW) blots of the pathogens proteome, probed with biotinylated C. burnetii genomic DNA. Confirmation of the identity of the DNA-binding protein as BCP (CBU_0963) was established by matrix-assisted laser desorption ionization-tandem time of flight mass spectrometry (MALDI-TOF/TOF MS). Recombinant Coxiella BCP (rBCP) was generated, and its DNA binding was demonstrated by two independent methods, including SW blotting and electrophoretic mobility shift assays (EMSAs). rBCP also demonstrated peroxidase activity in vitro that required thioredoxin-thioredoxin reductase (Trx-TrxR). Both the DNA-binding and peroxidase activities of rBCP were lost upon heat denaturation (100 degrees C, 10 min). Functional expression of Coxiella bcp was demonstrated by trans-complementation of an Escherichia coli bcp mutant, as evidenced by the strains ability to grow in an oxidative-stress growth medium containing tert-butyl hydroperoxide to levels that were indistinguishable from, or significantly greater than, those observed with its wild-type parental strain and significantly greater than bcp mutant levels (P < 0.05). rBCP was also found to protect supercoiled plasmid DNA from oxidative damage (i.e., nicking) in vitro. Maximal expression of the bcp gene coincided with the pathogens early (day 2 to 3) exponential-growth phase in an experiment involving synchronized infection of an epithelial (Vero) host cell line. Taken as a whole, the results show that Coxiella BCP binds DNA and likely serves to detoxify endogenous hydroperoxide byproducts of Coxiellas metabolism during intracellular replication.

Identification of Novel Small RNAs and Characterization of the 6S RNA of Coxiella burnetii

Coxiella burnetii, an obligate intracellular bacterial pathogen that causes Q fever, undergoes a biphasic developmental cycle that alternates between a metabolically-active large cell variant (LCV) and a dormant small cell variant (SCV). As such, the bacterium undoubtedly employs complex modes of regulating its lifecycle, metabolism and pathogenesis. Small RNAs (sRNAs) have been shown to play important regulatory roles in controlling metabolism and virulence in several pathogenic bacteria. We hypothesize that sRNAs are involved in regulating growth and development of C. burnetii and its infection of host cells. To address the hypothesis and identify potential sRNAs, we subjected total RNA isolated from Coxiella cultured axenically and in Vero host cells to deep-sequencing. Using this approach, we identified fifteen novel C. burnetii sRNAs (CbSRs). Fourteen CbSRs were validated by Northern blotting. Most CbSRs showed differential expression, with increased levels in LCVs. Eight CbSRs were upregulated (≥2-fold) during intracellular growth as compared to growth in axenic medium. Along with the fifteen sRNAs, we also identified three sRNAs that have been previously described from other bacteria, including RNase P RNA, tmRNA and 6S RNA. The 6S regulatory sRNA of C. burnetii was found to accumulate over log phase-growth with a maximum level attained in the SCV stage. The 6S RNA-encoding gene (ssrS) was mapped to the 5′ UTR of ygfA; a highly conserved linkage in eubacteria. The predicted secondary structure of the 6S RNA possesses three highly conserved domains found in 6S RNAs of other eubacteria. We also demonstrate that Coxiella’s 6S RNA interacts with RNA polymerase (RNAP) in a specific manner. Finally, transcript levels of 6S RNA were found to be at much higher levels when Coxiella was grown in host cells relative to axenic culture, indicating a potential role in regulating the bacterium’s intracellular stress response by interacting with RNAP during transcription.

Pentamidine inhibits Coxiella burnetii growth and 23S rRNA intron splicing in vitro

Coxiella burnetii is the bacterial agent of Q fever in humans. Acute Q fever generally manifests as a flu-like illness and is typically self-resolving. In contrast, chronic Q fever usually presents with endocarditis and is often life-threatening without appropriate antimicrobial therapy. Unfortunately, available options for the successful treatment of chronic Q fever are both limited and protracted (>18 months). Pentamidine, an RNA splice inhibitor used to treat fungal and protozoal infections, was shown to reduce intracellular growth of Coxiella by ca. 73% at a concentration of 1 microM (ca. 0.6 microg/mL) compared with untreated controls, with no detectable toxic effects on host cells. Bacterial targets of pentamidine include Cbu.L1917 and Cbu.L1951, two group I introns that disrupt the 23S rRNA gene of Coxiella, as demonstrated by the drugs ability to inhibit intron RNA splicing in vitro. Since both introns are highly conserved amongst all eight genotypes of the pathogen, pentamidine is predicted to be efficacious against numerous strains of C. burnetii. To our knowledge, this is the first report describing antibacterial activity for this antifungal/antiprotozoal agent.

Ribozyme Stability, Exon Skipping, and a Potential Role for RNA Helicase in Group I Intron Splicing by Coxiella Burnetii

The 23S rRNA gene of Coxiella burnetii, the agent of Q fever in humans, contains an unusually high number of conserved, selfish genetic elements, including two group I introns, termed Cbu.L1917 (L1917) and Cbu.L1951 (L1951). To better understand the role that introns play in Coxiellas biology, we determined the intrinsic stability time periods (in vitro half-lives) of the encoded ribozymes to be ∼15 days for L1917 and ∼5 days for L1951, possibly due to differences in their sizes (551 and 1,559 bases, respectively), relative degrees of compactness of the respective RNA structures, and amounts of single-stranded RNA. In vivo half-lives for both introns were also determined to be ∼11 min by the use of RNase protection assays and an Escherichia coli model. Intron RNAs were quantified in synchronous cultures of C. burnetii and found to closely parallel those of 16S rRNA; i.e., ribozyme levels significantly increased between days 0 and 3 and then remained stable until 8 days postinfection. Both 16S rRNA and ribozyme levels fell during the stationary and death phases (days 8 to 14). The marked stability of the Coxiella intron RNAs is presumably conferred by their association with ribosomes, a stoichiometric relationship that was determined to be one ribozyme, of either type, per 500 ribosomes. Inaccuracies in splicing (exon 2 skipping) were found to increase during the first 5 days in culture, with a rate of approximately one improperly spliced 23S rRNA per 1.3 million copies. The in vitro efficiency of L1917 intron splicing was significantly enhanced in the presence of a recombinant Coxiella RNA DEAD-box helicase (CBU_0670) relative to that of controls, suggesting that this enzyme may serve as an intron RNA splice facilitator in vivo.

A unique Coxiella burnetii lipoprotein involved in metal binding (LimB)

Coxiella burnetii is the bacterial agent of Q fever in humans. Here, we describe a unique, ~7.2 kDa, surface-exposed lipoprotein involved in metal binding which we have termed LimB. LimB was initially identified as a potential metal-binding protein on far-Western (FW) blots containing whole-cell lysate proteins when probed with nickel-coated horseradish peroxidase (Ni-HRP) and developed with a chemiluminescent HRP substrate. The corresponding identity of LimB as CBU1224a was established by matrix-assisted laser desorption ionization-tandem time-of-flight mass spectrometry. blast analyses with CBU1224a showed no significant similarity to sequences outside strains of C. burnetii. Additional in silico analyses revealed a putative 20 residue signal sequence with the carboxyl end demarcated by a potential lipobox (LSGC) whose Cys residue is predicted to serve as the N-terminal, lipidated Cys of mature LimB. The second residue of mature LimB is predicted to be Ala, an uncharged envelope localization residue. These features suggest that CBU1224a is synthesized as a prolipoprotein which is subsequently lipidated, secreted and anchored in the outer membrane. Mature LimB is predicted to contain 45 aa, of which there are 10 His and 5 Cys; both amino acids are frequently involved in binding transition metal cations. Recombinant LimB (rLimB) was generated and its Ni-HRP-binding activity demonstrated on FW blots. Ni-HRP binding by rLimB was inhibited by >95 % on FW blots done in the presence of EDTA, imidazole, Ni(2+) or Zn(2+), and roughly halved in the presence of Co(2+) or Fe(3+). The limB gene was maximally expressed at 3-7 days post-infection in Coxiella-infected Vero cells, coinciding with exponential phase growth. Two isoforms of LimB were detected on FW and Western blots, including a smaller (~7.2 kDa) species that was the predominant form in small cell variants and a larger isoform (~8.7 kDa) in large cell variants. LimB is Sarkosyl-insoluble, like many omps. The predicted surface location of LimB was verified by immunoelectron and immunofluorescence microscopy using anti-rLimB antibodies. Overall, the results suggest that LimB is a unique Coxiella lipoprotein that serves as a surface receptor for divalent metal cations and may play a role in acquiring at least one of these metals during intracellular growth.

The Intervening Sequence of Coxiella burnetii: Characterization and Evolution

The intervening sequence (IVS) of Coxiella burnetii, the agent of Q fever, is a 428-nt selfish genetic element located in helix 45 of the precursor 23S rRNA. The IVS element, in turn, contains an ORF that encodes a hypothetical ribosomal S23 protein (S23p). Although S23p can be synthesized in vitro in the presence of an engineered E. coli promoter and ribosome binding site, results suggest that the protein is not synthesized in vivo. In spite of a high degree of IVS conservation among different strains of C. burnetii, the region immediately upstream of the S23p start codon is prone to change, and the S23p-encoding ORF is evidently undergoing reductive evolution. We determined that IVS excision from 23S rRNA was mediated by RNase III, and IVS RNA was rapidly degraded, thereafter. Levels of the resulting 23S rRNA fragments that flank the IVS, F1 (~1.2 kb) and F2 (~1.7 kb), were quantified over C. burnetiis logarithmic growth phase (1–5 d). Results showed that 23S F1 quantities were consistently higher than those of F2 and 16S rRNA. The disparity between levels of the two 23S rRNA fragments following excision of IVS is an interesting phenomenon of unknown significance. Based upon phylogenetic analyses, IVS was acquired through horizontal transfer after C. burnetiis divergence from an ancestral bacterium and has been subsequently maintained by vertical transfer. The widespread occurrence, maintenance and conservation of the IVS in C. burnetii imply that it plays an adaptive role or has a neutral effect on fitness.

Streamlined preparation of genomic DNA in agarose plugs for pulsed-field gel electrophoresis

Genome analysis using pulsed-field gel electrophoresis (PFGE) has been used in applications ranging from typing bacterial strains to radiobiology to cancer research. While methods for running PFGE have been significantly improved since its invention, the method for preparing chromosomal DNA itself has remained essentially unchanged. This limits the applicability of PFGE, especially when analyses require many samples. We have streamlined sample preparation for routine applications of PFGE through the use of deep-well 48-well plates. Besides saving time, our protocol has the added advantage of reducing the volume of expensive reagents. Our improved protocol enables us to reduce throughput time and simplify the procedure, facilitating wider application of PFGE-based analyses in the laboratory.