Charged amino-acid residues in transmembrane domains of the plastidic ATP/ADP transporter from arabidopsis are important for transport efficiency, substrate specificity, and counter exchange properties.

Structure-function relationships of the plastidic ATP/ADP transporter from Arabidopsis thaliana have been determined using site-directed mutants at positions K155, E245, E385, and K527. These charged residues are found within highly conserved domains of homologous transport proteins from plants and bacteria and are located in predicted transmembrane regions. Mutants of K155 to K155E, K155R, or K155Q reduced ATP transport to values between 4 and 16% of wild-type uptake, whereas ADP transport was always less then 3% of the wild-type value. Site-directed mutations in which glutamate at positions 245 or 385 was replaced with lysine, abolished transport. However, conservative (E245D, E385D) or neutral (E245Q, E385Q) replacement at these two positions allowed transport. The fourth reciprocal exchange, K527E, also abolished uptake of both adenylates. K527R and K527Q were unable to transport ATP, but ADP transport remained at 35 and 27%, respectively, of the wild-type activity. There was a 70-fold decreased apparent affinity of K527R for ATP, but only a twofold decrease for ADP. The efflux of ATP, but not ADP, was also greatly reduced in K527R. These observations show strikingly that K527 plays a role in substrate specificity that is manifest in both the influx and efflux components of this antiporter.

Transformation of Rickettsia prowazekii to erythromycin resistance encoded by the Escherichia coli ereB gene.

Rickettsia prowazekii, the etiologic agent of epidemic typhus, is an obligate, intracytoplasmic, parasitic bacterium. Recently, the transformation of this bacterium via electroporation has been reported. However, in these studies identification of transformants was dependent upon either selection of an R. prowazekii rpoB chromosomal mutation imparting rifampin resistance or expression of the green fluorescent protein and flow cytometric analysis. In this paper we describe the expression in R. prowazekii of the Escherichia coli ereB gene. This gene codes for an erythromycin esterase that cleaves erythromycin. To the best of our knowledge, this is the first report of the expression of a nonrickettsial, antibiotic-selectable gene in R. prowazekii. The availability of a positive selection for rickettsial transformants is an important step in the characterization of genetic analysis systems in the rickettsiae.

Gene synthesis, bacterial expression and purification of the Rickettsia prowazekii ATP/ADP translocase.

The Rickettsia prowazekii ATP/ADP translocase (Tlc) is the first member of a new family of ATP/ADP exchangers that includes both prokaryotic and eukaryotic proteins. We optimized the codon usage for expression of tlc in Escherichia coli by means of gene synthesis, expressed the synthetic gene in E. coli, and purified a modified Tlc that contained a C-terminal tag of 10 consecutive histidine residues by immobilized metal affinity chromatography. Although codon usage in R. prowazekii is very different from E. coli, the optimization of the codon usage by itself was insufficient to improve expression. However, the change of the cloning vector from pET11a to pT7-5 led to a 3-10-fold increase in the specific ATP transport rate by cells expressing the synthetic construct. The authenticity of the purified protein was confirmed by N-terminal amino acid sequencing and a matrix assisted laser desorption/ionization mass spectrometry.

Rickettsia prowazekii transports UMP and GMP, but not CMP, as building blocks for RNA synthesis.

Rickettsia prowazekii, the etiological agent of epidemic typhus, is an obligate intracellular bacterium and is apparently unable to synthesize ribonucleotides de novo. Here, we show that as an alternative, isolated, purified R. prowazekii organisms transported exogenous uridyl- and guanylribonucleotides and incorporated these labeled precursors into their RNA in a rifampin-sensitive manner. Transport systems for nucleotides, which we have shown previously and show here are present in rickettsiae, have never been reported in free-living bacteria, and the usual nucleobase and nucleoside transport systems are absent in rickettsiae. There was a clear preference for the monophosphate form of ribonucleotides as the transported substrate. In contrast, rickettsiae did not transport cytidylribonucleotides. The source of rickettsial CTP appears to be the transport of UMP followed by its phosphorylation and the amination of intrarickettsial UTP to CTP by CTP synthetase. A complete schema of nucleotide metabolism in rickettsiae is presented that is based on a combination of biochemical, physiological, and genetic information.

Non-mitochondrial ATP transport.

Exchange of organelle ATP with cytosolic ADP through the ADP/ATP carrier is a well-characterized feature of mitochondrial metabolism. Obligate intracellular bacteria, such as Rickettsia prowazekii, and higher-plant plastids possess another type of adenylate transporter, which exchanges bacterial or plastidic ADP for ATP from the eukaryotic (host cell) cytoplasm. The bacterial and plastidic transporters are similar but do not share significant sequence similarities with the mitochondrial carrier. Recent molecular and biochemical studies are providing deeper insight into the functional and evolutionary relationships between the bacterial and the plant transport proteins.

Two nucleotide transport proteins in Chlamydia trachomatis, one for net nucleoside triphosphate uptake and the other for transport of energy.

The genome of Chlamydia trachomatis, one of the most prominent human pathogens, contains two structural genes coding for proteins, herein called Npt1Ct and Npt2Ct (nucleoside phosphate transporters 1 and 2 of C. trachomatis), exhibiting 68 and 61% similarity, respectively, to the ATP/ADP transporter from the intracellular bacterium Rickettsia prowazekii at the deduced amino acid level. Hydropathy analysis and sequence alignments suggested that both proteins have 12 transmembrane domains. The putative transporters were expressed as histidine-tagged proteins in Escherichia coli to study their biochemical properties. His10-Npt1Ct catalyzed ATP and ADP transport in an exchange mode. The apparent Km values were 48 (ATP) and 39 (ADP) microM. ATP and ADP transport was specific since AMP, GTP, CTP, UTP, dATP, dCTP, dGTP, and dTTP did not inhibit uptake. In contrast, His10-Npt2Ct transported all four ribonucleoside triphosphates with apparent Km values of 31 microM (GTP), 302 microM (UTP), 528 microM (CTP), and 1,158 microM (ATP). Ribonucleoside di- and monophosphates and deoxyribonucleotides were not substrates. The protonophore m-chlorocarbonylcyanide phenylhydrazone abolished uptake of all nucleoside triphosphates by Npt2Ct. This observation indicated that His10-Npt2Ct acts as a nucleosidetriphosphate/H+ symporter energized by the proton motive force across the Escherichia coli cytoplasmic membrane. We conclude that Npt1Ct provides chlamydiae with energy whereas Npt2Ct catalyzes the net uptake of ribonucleoside triphosphates required for anabolic reactions.

Membrane topology of the Rickettsia prowazekii ATP/ADP translocase revealed by novel dual pho-lac reporters.

Here, we report the construction and characterization of dual reporters, consisting of both an Escherichia coli alkaline phosphatase (AP) gene and an alpha-fragment of the beta-galactosidase (BG) gene, for studying membrane protein topology by the gene fusion approach. Each of the reporters, when fused to periplasmic domains of polytopic proteins, produces fusions with high AP activity and, when fused to cytoplasmic domains, produces fusions with high BG activity in E. coli strains capable of alpha-complementation. The dual nature of these reporters simplifies interpretation of data obtained with poorly expressed fusions and allows one to evaluate the reliability of topological data. Deleterious effects resulting from the cell's attempt to export the full-length BG are eliminated in this approach. We describe dual indicator plates that allow for discrimination between colonies bearing cytoplasmic fusions, periplasmic fusions, and no fusions. We have generated a set of fusions to the topologically well-studied lactose permease of E. coli and demonstrated that topological information generated by these new reporters is in good agreement with the existing model. We used this new methodology for the determination of membrane topology of the Rickettsia prowazekii ATP/ADP translocase (Tlc). Our results were in agreement with the proposed in silico topological model in which Tlc traverses the cytoplasmic membrane of E. coli 12 times with its N and C termini facing the cytoplasm.

The genome sequence of Rickettsia prowazekii and the origin of mitochondria.

We describe here the complete genome sequence (1,111,523 base pairs) of the obligate intracellular parasite Rickettsia prowazekii, the causative agent of epidemic typhus. This genome contains 834 protein-coding genes. The functional profiles of these genes show similarities to those of mitochondrial genes: no genes required for anaerobic glycolysis are found in either R. prowazekii or mitochondrial genomes, but a complete set of genes encoding components of the tricarboxylic acid cycle and the respiratory-chain complex is found in R. prowazekii. In effect, ATP production in Rickettsia is the same as that in mitochondria. Many genes involved in the biosynthesis and regulation of biosynthesis of amino acids and nucleosides in free-living bacteria are absent from R. prowazekii and mitochondria. Such genes seem to have been replaced by homologues in the nuclear (host) genome. The R. prowazekii genome contains the highest proportion of non-coding DNA (24%) detected so far in a microbial genome. Such non-coding sequences may be degraded remnants of 'neutralized' genes that await elimination from the genome. Phylogenetic analyses indicate that R. prowazekii is more closely related to mitochondria than is any other microbe studied so far.

Isolation and characterization of the dnaA gene of Rickettsia prowazekii.

The dnaA gene encoding the initiator protein of DNA replication was isolated from the obligate intracellular bacterium, Rickettsia prowazekii. Comparison of the deduced amino acid sequence of R. prowazekii DnaA with other bacterial DnaA proteins revealed extensive similarity. However, the rickettsial sequence is unique in the number of basic lysine residues found within a highly conserved portion of the putative DNA binding region, suggesting that the rickettsial protein may recognize a DNA sequence that differs from the consensus DnaA box sequence identified in other bacteria. Consensus DnaA box sequences, found upstream of many bacterial dnaA genes, were not identified upstream of rickettsial dnaA gene. In addition, gene organization within this region differed from that of other bacteria. The putative start of transcription of the rickettsial dnaA gene was localized to a site 522 nucleotides (nt) upstream of the DnaA start codon.

Transformation of Rickettsia prowazekii to rifampin resistance.

Rickettsia prowazekii, the causative agent of epidemic typhus, is an obligate intracellular parasitic bacterium that grows directly within the cytoplasm of the eucaryotic host cell. The absence of techniques for genetic manipulation hampers the study of this organism's unique biology and pathogenic mechanisms. To establish the feasibility of genetic manipulation in this organism, we identified a specific mutation in the rickettsial rpoB gene that confers resistance to rifampin and used it to demonstrate allelic exchange in R. prowazekii. Comparison of the rpoB sequences from the rifampin-sensitive (Rifs) Madrid E strain and a rifampin-resistant (Rifr) mutant identified a single point mutation that results in an arginine-to-lysine change at position 546 of the R. prowazekii RNA polymerase beta subunit. A plasmid containing this mutation and two additional silent mutations created in codons flanking the Lys-546 codon was introduced into the Rifs Madrid E strain of R. prowazekii by electroporation, and in the presence of rifampin, resistant rickettsiae were selected. Transformation, via homologous recombination, was demonstrated by DNA sequencing of PCR products containing the three mutations in the Rifr region of rickettsial rpoB. This is the first successful demonstration of genetic transformation of Rickettsia prowazekii and represents the initial step in the establishment of a genetic system in this obligate intracellular pathogen.

Occurrence of two plastidic ATP/ADP transporters in Arabidopsis thaliana L.--molecular characterisation and comparative structural analysis of similar ATP/ADP translocators from plastids and Rickettsia prowazekii.

Recently, we sequenced a cDNA clone from Arabidopsis thaliana L. encoding an ATP/ADP transporter protein (AATP1) located in the plastid envelope membrane. The deduced amino acid sequence of AATP1 exhibits a high degree of similarity (> 66%) to the ATP/ADP transporter from the obligate intracellular gram-negative bacterium Rickettsia prowazekii. Here we report a second plastidic ATP/ADP carrier from A. thaliana (AATP2). As deduced from the amino acid sequence, AATP2 exhibits 77.6% identity to AATP1 and 36% to the rickettsial protein. Hydropathy analysis indicates that all three translocators are highly hydrophobic membrane proteins, which exhibit marked similarities and differences. The AATP1 translocator lacks the sixth transmembrane domain that is present in AATP2 and the bacterial transporter in R. prowazekii. In contrast to AATP1 and the bacterial transport protein, only AATP2 exhibits a truncated C-terminal end. To compare the general biochemical properties of AATP2 with the known transport properties of AATP1 we cloned the entire AATP2 cDNA into plasmid pJT118, leading to the presence of an additional N-terminal histidine tag of 10 amino acids. For heterologous expression of His10-AATP2 we chose the Escherichia coli strain C43, which was reported recently to allow overproduction of eucaryotic membrane transport proteins. After transformation and subsequent induction by isopropylthio-2-D-galactopyranoside intact E. coli cells harbouring plasmid pJT118 showed import of radioactively labelled ATP and ADP. As deduced from a Lineweaver-Burk analysis His10-AATP2 exhibited apparent Km values for ATP and ADP of 22 microM and 20 microM, respectively. Import of ADP into His10-AATP2-expressing E. coli cells occurred at a rate of 24 nmol x mg protein(-1) x h(-1), which was about threefold faster than import of ATP. These biochemical characteristics are similar to transport properties of the heterologously expressed His10-AATP1 protein.

Nitric oxide-mediated inhibition of the ability of Rickettsia prowazekii to infect mouse fibroblasts and mouse macrophagelike cells.

The role of the nitric oxide synthase (NOS) pathway in inhibiting the ability of Rickettsia prowazekii to initially infect (invade) mouse cytokine-treated, fibroblastic L929 cells and macrophagelike RAW264.7 cells and the ability of nitric oxide (NO) to damage isolated rickettsiae were investigated. Substantial amounts of nitrite (a degradation product of NO) were produced and the initial rickettsial infection was suppressed in cultures of L929 cells treated with crude lymphokine preparations (LK) or with gamma interferon (IFN-gamma) plus tumor necrosis factor alpha (TNF-alpha) but not in L929 cell cultures treated with IFN-gamma alone or TNF-alpha alone. The NOS inhibitors N(G)-methyl-L-arginine and aminoguanidine both inhibited nitrite production and prevented the suppression of the initial rickettsial infection. Antibody-mediated neutralization of the IFN-gamma in the LK also inhibited both nitrite production and suppression of the initial rickettsial infection. Cultures of RAW264.7 cells treated with IFN-gamma plus lipopolysaccharide exhibited suppression of the initial rickettsial infection, and the suppression was relieved by aminoguanidine. Addition of oxyhemoglobin (a scavenger of extracellular NO) during the rickettsial infection alleviated the suppression of the initial rickettsial infection observed in appropriately treated L929 cells and RAW264.7 cells. In addition, the oxyhemoglobin restored the rickettsia-mediated, rapid killing of the treated RAW264.7 cells. Incubation of isolated rickettsiae with NO inhibited their ability to infect L929 and IFN-gamma-treated RAW264.7 cells and to rapidly kill IFN-gamma-treated RAW264.7 cells. In contrast, incubation of L929 cells with a solution that contained NO and/or degradation products of NO did not affect their ability to be infected by rickettsiae. The data are consistent with the hypothesis that NO released from appropriately stimulated potential host cells kills extracellular rickettsiae and thus prevents the rickettsiae from infecting the cells.

Transcriptional characterization of the Rickettsia prowazekii major macromolecular synthesis operon.

Recent studies have demonstrated that Rickettsia prowazekii can regulate transcription of selected genes at the level of initiation. However, little information concerning the existence of operons and coordinate gene regulation in this obligate intracellular parasitic bacterium is available. To address these issues, we have focused on the rpoD gene linkage group (greA-open reading frame 23 [ORF23]-dnaG-rpoD), which includes the rickettsial analog (ORF23-dnaG-rpoD) of the major macromolecular synthesis operon (MMSO). The rickettsial MMSO consists of an ORF coding for a protein of unknown function the structural genes for DNA primase (dnaG) and the major sigma factor of RNA polymerase (rpoD). RNase protection assays (RPA) were used to determine if these genes are organized into an operon controlled by multiple promoters and the quantities of transcripts produced by these genes relative to each other. RPA with a probe spanning the 270-base greA-ORF23 intervening region identified a putative transcriptional promoter within the intervening sequence. Multiple RPA probes spanning the next 4,041 bases of the linkage group demonstrated the presence of a continuous transcript and thus the existence of an operon. A probe spanning the dnaG-rpoD region revealed that two additional mRNA fragments were also protected, which enabled us to identify additional putative promoters for rpoD within dnaG. Primer extension determined that the 5' ends of the three transcripts consist separately of adenine (located 227 bases upstream of ORF23) and uracil and adenine (located 336 and 250 bases upstream of rpoD, respectively). Quantitation of transcripts produced by the three ORFs determined the relative amounts of transcripts (ORF23 to dnaG to rpoD) to be 1:2.7:5.1.

Transcriptional regulation of the gltA and TLC genes in Rickettsia prowazekii growing in a respiration-deficient host cell.

The regulation of the citrate synthase (gltA) and ATP/ADP translocase (tlc) genes of the obligate intracellular bacterium, Rickettsia prowazekii, was analyzed in rickettsia-infected respiration-deficient G14 cells. The level of the gltA mRNAII and the tlc mRNA was much lower in the total RNA isolated from the infected G14 cells grown in 1 g/l glucose (low glucose, GL) medium than in that from infected G14 cells grown in 4.5 g/l glucose (high glucose, GH) medium. However, the level of the gltA mRNAI relative to 16 S rRNA was the same in GL and GH media. An increase in the level of the gltA mRNAII and the tlc mRNA could be observed as early as 2 hrs after shifting from GL to GH medium. We conclude that, under these experimental conditions, the tlc promoter and the gltA promoter P2, but not gltA promoter P1, were transcriptionally regulated.

Transcriptional regulation in the obligate intracytoplasmic bacterium Rickettsia prowazekii.

Transcriptional regulation was demonstrated in Rickettsia prowazekii, an obligate intracytoplasmic bacterium. The level of citrate synthase (gltA) mRNA II, from promoter P2, was greater in the total RNA isolated from heavily infected L929 cells than in moderately infected L929 cells; conversely, the level of ATP/ADP translocase (tlc) mRNA was greater in moderately infected cells. The level of gltA mRNA I, from promoter P1, did not change under these conditions. The chemical half-lives of gltA mRNA II and tlc mRNA under these conditions were very similar.

Transcriptional analysis of the 16s rRNA gene in Rickettsia prowazekii.

The control of rRNA synthesis in the etiological agent of epidemic typhus, Rickettsia prowazekii, a slowly growing obligate intracytoplasmic bacterium, was investigated. Transcription of the rickettsial 16S rRNA gene (rrs), of which there is only a single copy, was controlled by a single promoter region, and the site for the initiation of transcription (base A) was found 117 bp upstream of the rrs coding region for the mature product. The promoter region contained an Escherichia coli promoter-like sequence, TTGACA-N17-TATAAC, centered 139 bp upstream of the coding region for the mature product. To investigate whether transcription of the rickettsial rrs responds to amino acid starvation conditions, total RNA was isolated from R. prowazekii-infected mouse L929 cells with or without methionine starvation. The level of newly synthesized 16S rRNA precursors in R. prowazekii, as analyzed by ribonuclease protection assays, decreased significantly after methionine starvation for 6 h and then recovered within 12 h after the addition of methionine. The chemical half-lives of the 16S rRNA precursors in the methionine-starved rickettsiae did not differ significantly from those in the normal rickettsiae. These results suggest that R. prowazekii regulates transcription of the rrs in response to amino acid starvation conditions.

Instability of Rickettsia prowazekii RNA polymerase-promoter complexes.

The Rickettsia prowazekii sigma factor was overexpressed, purified, and used to reconstitute RNA polymerase holoenzyme species. R. prowazekii RNA polymerase-promoter complexes were unstable and remained dissociable and heparin sensitive under conditions in which the corresponding Escherichia coli complexes were not. The R. prowazekii core played the major role in determining heparin sensitivity.

The citrate synthase-encoding gene of Rickettsia prowazekii is controlled by two promoters.

The transcripts of the citrate synthase-encoding gene (gltA) in Rickettsia prowazekii (Rp), an obligate intracellular parasitic bacterium, were analyzed by RNase protection (RP), primer extension (PE) and in vitro transcription assays. Analysis of the 5' end of the gltA mRNA by RP and PE assays revealed that there were two gltA mRNAs with the 5' ends located at 16 bp and 307 bp upstream from the gltA coding region. Since these two mRNAs might represent two species of mRNA transcribed from two different promoters or a single transcript that was processed to give two mRNAs, an in vitro transcription analysis with purified Rp RNA polymerase (RNAP) was performed to distinguish these two possibilities. Purified Rp RNAP catalyzed the formation of two transcripts initiated from the same nucleotides indicated by RP and PE. Sequence analysis identified Escherichia coli (Ec) promoter-like sequences immediately upstream from both transcription start points (tsp). The first promoter (promoter P1) had the core sequence TTCTAA-N17-TATACT, was 6 bp upstream from the tsp (base A) and was centered at 37 bp upstream from the coding region. The second promoter (promoter P2) had the core sequence ATGAAA-N17-TAAAGT, was 7 bp upstream from the tsp (base T) and was centered at 329 bp upstream from the coding region. This is the first demonstration of multiple promoters in this obligate intracellular parasite which has implications concerning transcriptional regulation.

Unusual organization of the rRNA genes in Rickettsia prowazekii.

We describe here the organization of the rRNA genes in Rickettsia prowazekii. In this organism, the 23S and the 5S rRNA genes are tightly linked to each other, whereas the 16S rRNA gene is separated from this cluster. The 23S-5S unit is preceded by the methionyl-tRNAfMet formyltransferase gene.

Rickettsia prowazekii, ribosomes and slow growth.

Some bacteria, such as Rickettsia prowazekii, grow slowly, not with anticipation of a future feast, but because it is evolutionarily advantageous to do so. This creates apparent paradoxes for understanding their physiology and biochemistry. These rickettsiae have a ribosome concentration higher than expected if these ribosomes support translation at rates comparable to those in Escherichia coli.