Isolation and characterization of the Rickettsia prowazekii gene encoding the flavoprotein subunit of succinate dehydrogenase

Germline SDHA mutations in children and adults with cancer

Mutations in succinate dehydrogenase complex genes predispose to familial paraganglioma-pheochromocytoma syndrome (FPG) and gastrointestinal stromal tumors (GIST). Here we describe cancer patients undergoing agnostic germline testing at Memorial Sloan Kettering Cancer Center and found to harbor germline SDHA mutations. Using targeted sequencing covering the cancer census genes, we identified 10 patients with SDHA germline mutations. Cancer diagnoses for these patients carrying SDHA germline mutations included neuroblastoma (n = 1), breast (n = 1), colon (n = 1), renal (n = 1), melanoma and uterine (n = 1), prostate (n = 1), endometrial (n = 1), bladder (n = 1), and gastrointestinal stromal tumor (GIST) (n = 2). Immunohistochemical staining and assessment of patient tumors for second hits and loss of heterozygosity in SDHA confirmed GIST as an SDHA-associated tumor and suggests SDHA germline mutations may be a driver in neuroblastoma tumorigenesis.

Complex II from phototrophic purple bacterium Rhodoferax fermentans displays rhodoquinol-fumarate reductase activity

It has long been accepted that bacterial quinol-fumarate reductase (QFR) generally uses a low-redox-potential naphthoquinone, menaquinone (MK), as the electron donor, whereas mitochondrial QFR from facultative and anaerobic eukaryotes uses a low-redox-potential benzoquinone, rhodoquinone (RQ), as the substrate. In the present study, we purified novel complex II from the RQ-containing phototrophic purple bacterium, Rhodoferax fermentans that exhibited high rhodoquinol-fumarate reductase activity in addition to succinate-ubiquinone reductase activity. SDS/PAGE indicated that the purified R. fermentans complex II comprises four subunits of 64.0, 28.6, 18.7 and 17.5 kDa and contains 1.3 nmol heme per mg protein. Phylogenetic analysis and comparison of the deduced amino acid sequences of R. fermentans complex II with pro/eukaryotic complex II indicate that the structure and the evolutional origins of R. fermentans complex II are closer to bacterial SQR than to mitochondrial rhodoquinol-fumarate reductase. The results strongly indicate that R. fermentans complex II and mitochondrial QFR might have evolved independently, although they both utilize RQ for fumarate reduction.

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.

Genomic rearrangements during evolution of the obligate intracellular parasite Rickettsia prowazekii as inferred from an analysis of 52015 bp nucleotide sequence

In this study a description is given of the sequence and analysis of 52 kb from the 1.1 Mb genome of Rickettsia prowazekii, a member of the alpha-Proteobacteria. An investigation was made of nucleotide frequencies and amino acid composition patterns of 41 coding sequences, distributed in 10 genomic contigs, of which 32 were found to have putative homologues in the public databases. Overall, the coding content of the individual contigs ranged from 59 to 97%, with a mean of 81%. The genes putatively identified included genes involved in the biosynthesis of nucleotides, macromolecules and cell wall structures as well as citric acid cycle component genes. In addition, a putative identification was made of a member of the regulatory response family of two-component signal transduction systems as well as a gene encoding haemolysin. For one gene, the homologue of metK, an internal stop codon was discovered within a region that is otherwise highly conserved. Comparisons with the genomic structures of Escherichia coli, Haemophilus influenzae and Bacillus subtilis have revealed several atypical gene organization patterns in the R. prowazekii genome. For example, R. prowazekii was found to have a unique arrangement of genes upstream of dnaA in a region that is highly conserved among other microbial genomes and thought to represent the origin of replication of a primordial replicon. The results presented in this paper support the hypothesis that the R. prowazekii genome is a highly derived genome and provide examples of gene order structures that are unique for the Rickettsia.

Cloning and characterization of the cDNA encoding human biliverdin-IX alpha reductase

Biliverdin reductase is classified into two isoforms in substrate specificity; biliverdin-IX alpha reductase and biliverdin-IX beta reductase with a molecular mass of 22 kDa and 34-42 kDa, respectively. We have cloned the cDNA encoding human biliverdin-IX alpha reductase from MOLT4 cDNA library. The cDNA of 1146 bp in nucleotide length contained an entire reading frame coding 296 amino acid residues. The NADH/NADPH binding consensus sequence was found in the amino-terminal region. Comparison between human and rat biliverdin-IX alpha reductases showed 82.8% identity in amino acid sequences and 80.3% identity in the coding nucleotides. The amino acid sequence of human biliverdin-IX alpha reductase showed no significant homology to that of human biliverdin-IX beta reductase. Northern blot analysis of poly(A) RNA from eight different human tissues revealed that the reductase mRNA was abundant in the brain, lung and pancreas but not in the liver. The distribution pattern of biliverdin-IX alpha message was different from that of heme oxygenase activity which is known to be high in the liver and to be low in the heart and lung.

L-aspartate oxidase from Escherichia coli. I. Characterization of coenzyme binding and product inhibition

This paper reports the biochemical characterization of the flavoprotein L-aspartate oxidase from Escherichia coli. Modification of a previously published procedure allowed overexpression of the holoenzyme in an unproteolysed form. L-Aspartate oxidase is a monomer of 60 kDa containing 1 mol of noncovalently bound FAD/mol protein. A polarographic and two spectrophotometric coupled assays have been set up to monitor the enzymatic activity continuously. L-Aspartate oxidase was subjected to product inhibition since iminoaspartate, which results from the oxidation of L-aspartate, binds to the enzyme with a dissociation constant (Kd) equal to 1.4 microM. The enzyme binds FAD by a simple second-order process with Kd 0.67 microM. Site-directed mutagenesis of the residues E43, G44, S45, F47 and Y48 located in the putative binding site of the isoallossazinic portion of FAD reduces the affinity for the coenzyme.

Genes encoding the same three subunits of respiratory complex II are present in the mitochondrial DNA of two phylogenetically distant eukaryotes

Although mitochondrial DNA is known to encode a limited number (<20) of the polypeptide components of respiratory complexes I, III, IV, and V, genes for components of complex II [succinate dehydrogenase (ubiquinone); succinate:ubiquinone oxidoreductase, EC 1.3.5.1] are conspicuously lacking in mitochondrial genomes so far characterized. Here we show that the same three subunits of complex II are encoded in the mitochondrial DNA of two phylogenetically distant eukaryotes, Porphyra purpurea (a photosynthetic red alga) and Reclinomonas americana (a heterotrophic zooflagellate). These complex II genes, sdh2, sdh3, and sdh4, are homologs, respectively, of Escherichia coli sdhB, sdhC, and sdhD. In E. coli, sdhB encodes the iron-sulfur subunit of succinate dehydrogenase (SDH), whereas sdhC and sdhD specify, respectively, apocytochrome b558 and a hydrophobic 13-kDa polypeptide, which together anchor SDH to the inner mitochondrial membrane. Amino acid sequence similarities indicate that sdh2, sdh3, and sdh4 were originally encoded in the protomitochondrial genome and have subsequently been transferred to the nuclear genome in most eukaryotes. The data presented here are consistent with the view that mitochondria constitute a monophyletic lineage.

Mutation of a nuclear succinate dehydrogenase gene results in mitochondrial respiratory chain deficiency

We now report a mutation in the nuclear-encoded flavoprotein (Fp) subunit gene of the succinate dehydrogenase (SDH) in two siblings with complex II deficiency presenting as Leigh syndrome. Both patients were homozygous for an Arg554Trp substitution in the Fp subunit. Their parents (first cousins) were heterozygous for the mutation that occurred in a conserved domain of the protein and was absent from 120 controls. The deleterious effect of the Arg to Trp substitution on the catalytic activity of SDH was observed in a SDH- yeast strain transformed with mutant Fp cDNA. The Fp subunit gene is duplicated in the human genome (3q29; 5p15), with only the gene on chromosome 5 expressed in human-hamster somatic cell hybrids. This is the first report of a nuclear gene mutation causing a mitochondrial respiratory chain deficiency in humans.

Characterization of the succinate dehydrogenase-encoding gene cluster (sdh) from the rickettsia Coxiella burnetii

We have identified and sequenced four genes that encode the protein subunits comprising the succinate dehydrogenase enzyme complex (Sdh) of the rickettsia Coxiella burnetii. The Sdh-encoding gene cluster (sdhCDAB) begins 3326 bp upstream from the citrate synthase-encoding gene (gltA) start codon and is read with opposite polarity. An open reading frame encoding the N-terminal 280 amino acids (aa) of 2-oxoglutarate dehydrogenase (SucA) begins 24 bp downstream from the stop codon of the gene specifying the iron-sulfur subunit (sdhB) of Sdh. The deduced aa sequence of Sdh subunits and the N-terminal portion of SucA revealed significant aa identity with the Esherichia coli homologues ranging from a low of 36.6% for SdhD to a high of 61.2% for SdhA and SdhB. Primer extension identified transcription start points (tsp) for sdh and sucA. The region upstream from the sdh tsp, but not the sucA tsp, displayed homology to promoter consensus sequences of E. coli. Further evidence that sucA transcription can occur independent of sdh transcription was provided by demonstrating that a TnphoA insertion disrupting sdhB had no effect on the production of SucA by an E. coli cell-extract-directed in vitro transcription/translation system. The plasmid clone pLPM60, which carries the C. burnetii sdhCDAB coding and upstream regulatory regions, rescued an E. coli sdhA mutant (MOB252), indicating functional expression of the rickettsial locus. A cell extract of MOB252 transformed with pLPM60 showed a sixfold greater level of Sdh enzyme activity over the E. coli wild type. A plasmid clone lacking the sdh upstream regulatory region did not complement nor produce sdh mRNA by dot blot analysis.(ABSTRACT TRUNCATED AT 250 WORDS)