Ian G. Charles

Evaluation of Salmonella typhimurium strains harbouring defined mutations in htrA and aroA in the murine salmonellosis model

Derivatives of the mouse-virulent Salmonella typhimurium strain SL1344 were constructed harbouring defined mutations in htrA, aroA or htrA aroA combined. When administered orally or intravenously to BALB/c mice, all the mutants were found to be highly attenuated. All mutants were able to confer significant protection against lethal challenge with SL1344 after a single oral dose of live organisms. SL1344 htrA mutants persisted in livers and spleens at a lower level than SL1344 aroA mutants after intravenous administration. SL1344 htrA aroA mutants persisted at an even lower level and were cleared from the livers and spleens of mice within 21 days of intravenous administration. Thus htrA and htrA aroA mutants can be considered as potential oral vaccines against salmonellosis.

Nitric oxide induces coupling of mitochondrial signalling with the endoplasmic reticulum stress response

Nitric oxide (NO) is a pleiotropic signalling molecule that binds to cytochrome c oxidase (complex IV) reversibly and in competition with oxygen. This action of NO has both physiological and pathophysiological consequences. Here we report that endogenously generated NO, which disrupts the respiratory chain, may cause changes in mitochondrial calcium flux. This induces cleavage of the endoplasmic reticulum (ER) stress-regulated transcription factor p90 ATF6 into an active p50 form. Cleavage depends on a calcium-dependent serine protease through a regulated intramembrane proteolysis (RIP) process. p50 ATF6 then translocates to the nucleus to upregulate expression of the ER-resident molecular chaperone, glucose-regulated protein 78 (Grp78). The increase in Grp78 provides significant cytoprotection against toxic agents, including thapsigargin, a selective ER calcium–ATPase inhibitor. Cytoprotection is abolished after treatment with cyclosporin A (CsA), which disrupts mitochondrial calcium signalling, or with the calcium chelator BAPTA-AM. The NO-mediated ER stress response is diminished in rho0 cells devoid of mitochondrial DNA, consistent with our evidence that NO-dependent mitochondrial disruption is coupled to the ER stress response.

Nitric oxide upregulates expression of DNA-PKcs to protect cells from DNA-damaging anti-tumour agents

Nitric-oxide synthase (NOS) activity has been detected in many human tumours, although its function is unclear. Here we show that exposure of cells to nitric oxide (NO) results in a 4–5-fold increase in expression of the DNA-dependent protein-kinase catalytic subunit (DNA-PKcs), one of the key enzymes involved in repairing double-stranded DNA breaks. This NO-mediated increase in enzymatically active DNA-PK not only protects cells from the toxic effects of NO, but also provides crossprotection against clinically important DNA-damaging agents, such as X-ray radiation, adriamycin, bleomycin and cisplatin. The NO-mediated increase in DNA-PKcs described here demonstrates the presence of a new and highly effective NO-mediated mechanism for DNA repair.

Recombinant Bordetella pertussis pertactin (P69) from the yeast Pichia pastoris: high-level production and immunological properties

Acellular whooping cough vaccines are based on pertussis toxoid but their effectiveness may be increased by the addition of other Bordetella pertussis antigens. We expressed the immunogenic outer membrane protein pertactin (P69) from B. pertussis to high levels in multi-copy transformants of the industrial yeast Pichia pastoris. In high-density fermentations, engineered P. pastoris yielded greater than 3 g of the protein per litre of culture. Purified recombinant pertactin was able to stimulate the incomplete protection afforded by toxoid to the level of the whole-cell vaccine, as shown by the Kendrick test, supporting its inclusion in future acellular vaccines.

The Mycobacterium tuberculosis shikimate pathway genes : evolutionary relationship between biosynthetic and catabolic 3-dehydroquinases

SummaryThe Mycobacterium tuberculosis shikimate pathway genes designated aroB and aroQ encoding 3-dehydroquinate synthase and 3-dehydroquinase, respectively were isolated by molecular cloning and their nucleotide sequences determined. The deduced dehydroquinate synthase amino acid sequence from M. tuberculosis showed high similarity to those of equivalent enzymes from prokaryotes and filamentous fungi. Surprisingly, the deduced M. tuberculosis 3-dehydroquinase amino acid sequence showed no similarity to other characterised prokaryotic biosynthetic 3-dehydroquinases (bDHQases). A high degree of similarity was observed, however, to the fungal catabolic 3-dehydroquinases (cDHQases) which are active in the quinic acid utilisation pathway and are isozymes of the fungal bDHQases. This finding indicates a common ancestral origin for genes encoding the catabolic dehydroquinases of fungi and the biosynthetic dehydroquinases present in some prokaryotes. Deletion of genes encoding shikimate pathway enzymes represents a possible approach to generation of rationally attenuated strains of M. tuberculosis for use as live vaccines.

Crystal Structure of SANOS, a Bacterial Nitric Oxide Synthase Oxygenase Protein from Staphylococcus aureus

Prokaryotic genes related to the oxygenase domain of mammalian nitric oxide synthases (NOSs) have recently been identified. Although they catalyze the same reaction as the eukaryotic NOS oxygenase domain, their biological function(s) are unknown. In order to explore rationally the biochemistry and evolution of the prokaryotic NOS family, we have determined the crystal structure of SANOS, from methicillin-resistant Staphylococcus aureus (MRSA), to 2.4 A. Haem and S-ethylisothiourea (SEITU) are bound at the SANOS active site, while the intersubunit site, occupied by the redox cofactor tetrahydrobiopterin (H(4)B) in mammalian NOSs, has NAD(+) bound in SANOS. In common with all bacterial NOSs, SANOS lacks the N-terminal extension responsible for stable dimerization in mammalian isoforms, but has alternative interactions to promote dimer formation.

Cloning, nucleotide sequence and heterologous expression of the protective outer-membrane protein P.68 pertactin from Bordetella bronchiseptica.

The prn gene encoding the 68 kDa protective outer-membrane protein of Bordetella bronchiseptica (P.68 pertactin) was cloned, sequenced and expressed in Escherichia coli. The gene was isolated by DNA:DNA hybridization experiments using a radioactively-labelled fragment of the homologous prn gene from Bordetella parapertussis. DNA sequence analysis reveals that the gene is capable of encoding a protein with a molecular mass of 93996 Da (P.94); this precursor molecule is processed to form the P.68 antigen on the surface of B. bronchiseptica. Heterologous expression of the full-length gene encoding P.94 in Escherichia coli results in similar processing, with the P.68 antigen targeted to the bacterial outer membrane. Comparison of P.94 with the P.93 and P.95 precursors, encoding homologous proteins from Bordetella pertussis and B. parapertussis, shows a high degree (greater than 90%) of homology. The major differences between all three proteins occur in the number of repeats of the two families (Gly-Gly-Xaa-Xaa-Pro)n and (Pro-Gln-Pro)n of reiterated sequence motifs.

Isolation, characterization and nucleotide sequences of the aroC genes encoding chorismate synthase from Salmonella typhi and Escherichia coli

The aroC genes from Salmonella typhi and Escherichia coli, encoding 5-enolpyruvylshikimate-3-phosphate phospholyase (chorismate synthase) were cloned in E. coli and their DNA sequences were determined. The aroC gene from S. typhi was isolated from a cosmid gene bank by complementation of an E. coli aroC mutant. The corresponding E. coli gene was isolated from a pBR322 gene bank by colony hybridization using DNA encoding the aroC gene from S. typhi as a hybridization probe. Analysis of the nucleotide sequence revealed that both genes have an open reading frame capable of encoding proteins comprising 361 amino acids. The calculated molecular mass of the protein from S. typhi is 39,108 Da while that of the protein from E. coli is 39,138 Da. Homology is particularly strong between the coding regions of the genes: 95% when protein sequences are compared, and 83% when DNA sequences are examined. Use of a deletion variant of the E. coli aroC gene demonstrates that the C-terminal 36 amino acids are not essential for the correct folding or functional activity of the chorismate synthase enzyme.

Functional analysis of the GTPases EngA and YhbZ encoded by Salmonella typhimurium

The S. typhimurium genome encodes proteins, designated EngA and YhbZ, which have a high sequence identity with the GTPases EngA/Der and ObgE/CgtAE of Escherichia coli. The wild‐type activity of the E. coli proteins is essential for normal ribosome maturation and cell viability. In order to characterize the potential involvement of the Salmonella typhimurium EngA and YhbZ proteins in ribosome biology, we used high stringency affinity chromatography experiments to identify strongly binding ribosomal partner proteins. A combination of biochemical and microcalorimetric analysis was then used to characterize these protein:protein interactions and quantify nucleotide binding affinities. These experiments show that YhbZ specifically interacts with the pseudouridine synthase RluD (KD = 2 μM and 1:1 stoichiometry), and we show for the first time that EngA can interact with the ribosomal structural protein S7. Thermodynamic analysis shows both EngA and YhbZ bind GDP with a higher affinity than GTP (20‐fold difference for EngA and 3.8‐fold for YhbZ), and that the two nucleotide binding sites in EngA show a 5.3‐fold difference in affinity for GDP. We report a fluorescence assay for nucleotide binding to EngA and YhbZ, which is suitable for identifying inhibitors specific for this ligand‐binding site, which would potentially inhibit their biological functions. The interactions of YhbZ with ribosome structural proteins that we identify may demonstrate a previously unreported additional function for this class of GTPase: that of ensuring delivery of rRNA modifying enzymes to the appropriate region of the ribosome.

The Affinity of a Major Ca2+ Binding Site on GRP78 Is Differentially Enhanced by ADP and ATP

GRP78 is a major protein regulated by the mammalian endoplasmic reticulum stress response, and up-regulation has been shown to be important in protecting cells from challenge with cytotoxic agents. GRP78 has ATPase activity, acts as a chaperone, and interacts specifically with other proteins, such as caspases, as part of a mechanism regulating apoptosis. GRP78 is also reported to have a possible role as a Ca2+ storage protein. In order to understand the potential biological effects of Ca2+ and ATP/ADP binding on the biology of GRP78, we have determined its ligand binding properties. We show here for the first time that GRP78 can bind Ca2+, ATP, and ADP, each with a 1:1 stoichiometry, and that the binding of cation and nucleotide is cooperative. These observations do not support the hypothesis that GRP78 is a dynamic Ca2+ storage protein. Furthermore, we demonstrate that whereas Mg2+ enhances GRP78 binding to ADP and ATP to the same extent, Ca2+ shows a differential enhancement. In the presence of Ca2+, the KD for ATP is lowered ∼11-fold, and the KD for ADP is lowered around 930-fold. The KD for Ca2+ is lowered ∼40-fold in the presence of ATP and around 880-fold with ADP. These findings may explain the biological requirement for a nucleotide exchange factor to remove ADP from GRP78. Taken together, our data suggest that the Ca2+-binding property of GRP78 may be part of a signal transduction pathway that modulates complex interactions between GRP78, ATP/ADP, secretory proteins, and caspases, and this ultimately has important consequences for cell viability.