Polymorphic variants of LIGHT (TNF superfamily-14) alter receptor avidity and bioavailability

The TNF superfamily member homologous to lymphotoxins, exhibits inducible expression, and competes with HSV glycoprotein D for herpesvirus entry mediator (HVEM), a receptor expressed by T lymphocytes (LIGHT) [TNF superfamily (SF)-14], is a key cytokine that activates T cells and dendritic cells and is implicated as a mediator of inflammatory, metabolic, and malignant diseases. LIGHT engages the lymphotoxin-beta receptor (LTbetaR) and HVEM (TNFRSF14), but is competitively limited in activating these receptors by soluble decoy receptor-3 (DcR3; TNFRSF6B). Two variants in the human LIGHT alter the protein at E214K (rs344560) in the receptor-binding domain and S32L (rs2291667) in the cytosolic domain; however, the functional impact of these polymorphisms is unknown. A neutralizing Ab failed to bind the LIGHT-214K variant, indicating this position as a part of the receptor-binding region. Relative to the predominant reference variant S32/E214, the other variants showed altered avidity with LTbetaR and less with HVEM. Heterotrimers of the LIGHT variants decreased binding avidity to DcR3 and minimized the inhibitory effect of DcR3 toward LTbetaR-induced activation of NF-kappaB. In patients with immune-mediated inflammatory diseases, such as rheumatoid arthritis, DcR3 protein levels were significantly elevated. Immunohistochemistry revealed synoviocytes as a significant source of DcR3 production, and DcR3 hyperexpression is controlled by posttranscriptional mechanisms. The increased potential for LTbetaR signaling, coupled with increased bioavailability due to lower DcR3 avidity, provides a mechanism of how polymorphic variants in LIGHT could contribute to the pathogenesis of inflammatory diseases.

Pulsed field gel electrophoresis reveals chromosome length differences between strains of Cladosporium fulvum (syn. Fulvia fulva)

Methods are described for the electrophoretic separation of chromosome-sized DNA molecules from the fungal tomato pathogen Cladosporium fulvum (syn. Fulvia fulva). Using a hexagonal electrode array and switching times of 75 min at 45 V for 14 days, nine bands could be resolved. By comparison with co-electrophoresed Aspergillus nidulans chromosomal DNA (which was resolved into seven bands), the sizes of the C. fulvum bands are estimated to be between 1.9 Mb and 5.4 Mb. The two largest bands are believed to be doublets, giving a minimum genome size of 44 Mb. Cloned probes for the ribosomal DNA repeat, an anonymous single copy fragment and a newly discovered retrotransposon were hybridized to blots of the pulsed field gels, demonstrating the use of this technique for genomic mapping. Most strains of C. fulvum had an identical pattern of bands. Two strains exhibited two polymorphisms which could be due to a translocation.

An evolutionary relationship between the ColE5-099 and the ColE9-J plasmids revealed by nucleotide sequencing

The nucleotide sequence of a 1124 bp fragment of the ColE5-099 plasmid which encodes colicin E5 immunity, a lys gene involved in colicin release from the host cell, and the 3' end of the colicin E5 structural gene has been determined. Open reading frames corresponding to the three genes have been located by analogy with similar sequences from other E colicin plasmids. The location of these open reading frames corresponds with the position of the genes as determined by subcloning and transposon mutagenesis. The amino acid sequence of the carboxy-terminal 107 amino acid residues of the colicin E5 gene shows no homology with any other E colicin, suggesting a different mode of action in killing sensitive cells. A comparison of the nucleotide sequence of this region of the ColE5-099 plasmid with that of the equivalent region of the ColE9-J plasmid suggests a close evolutionary relationship between these two plasmids.

Electron-paramagnetic-resonance and magnetic-circular-dichroism studies on the formate dehydrogenase-nitrate reductase particle from Pseudomonas aeruginosa

The membrane-bound respiratory particle complex of Pseudomonas aeruginosa, which reduces nitrate to nitrite using formate as the electron donor, was prepared and characterized by e.p.r. and low-temperature magnetic c.d. (m.c.d.) spectroscopy. The particle complex has two enzymic components, namely nitrate reductase (NiR) and formate dehydrogenase (FDH), which are multi-centred proteins containing molybdenum, iron-sulphur clusters and cytochrome. By using results from work on the purified extracted enzymes NiR and FDH to aid in the assignment, it has been possible to observe spectroscopically all the components of the electron-transfer chain in the intact particle. This led to a proposal for the organization of the metal components of the FDH-NiR chain. Molybdenum ions are at opposite ends of the chain and interact with, respectively, the formate-CO2 couple and the nitrate-nitrite couple. The molybdenum ion at the low-potential end of the chain passes electrons to cytochrome b of FDH, a bishistidine-co-ordinated haem with unusual steric restraint at the iron. The next component is a [4Fe-4S] cluster. This comprises all the components of FDH. Electrons are passed to the molybdenum of NiR via a number, probably two, of [4Fe-4S] clusters. No evidence has been found in this work for the presence of a quinone to mediate electron transfer between FDH and NiR. Cytochrome c appears to be able to feed electrons into the chain at the level of one of the [4Fe-4S] centres of NiR.

Purification and properties of formate dehydrogenase from Pseudomonas aeruginosa. Electron-paramagnetic-resonance studies on the molybdenum centre

Formate dehydrogenase from Pseudomonas aeruginosa contains molybdenum, a [4Fe-4S] cluster and cytochrome b. This paper reports the detection of molybdenum as Mo(V) by e.p.r. spectroscopy. In order to generate Mo(V) signals, addition of amounts of excess formate varying between 10- and 50-fold over enzyme, followed by 200-fold excess of sodium dithionite, were used. Two Mo(V) species were observed. One, the major component, has g1 = 2.012, g2 = 1.985 and g3 = 1.968, appeared at low concentrations of formate and increased linearly in intensity with increasing concentrations of formate up to 25-fold excess over the enzyme. At higher formate concentration this signal disappeared. The appearance and disappearance of this Mo(V) signal seems to parallel the state of reduction of the [4Fe-4S] clusters. A second, minor, Mo(V) species with g-values g1 = 1.996, g2 = 1.981 and g3 = 1.941 appears at a constant level during the formate-dithionite titration. No evidence has been obtained for nuclear hyperfine coupling to protons. The major Mo(V) species has unusual e.p.r. signals compared with other molybdenum-containing enzymes, except for that observed in the formate dehydrogenase from Methanobacterium formicicum [Barber, Siegel, Schauer, May & Ferry (1983) J. Biol. Chem. 258, 10839-10845]. The present work suggests that the enzyme is acting as a CO2 reductase, with dithionite as an electron donor to a [4Fe-4S] cluster, which in turn donates electrons to molybdenum, producing a Mo(V) species with CO2 bound to the metal.

Purification and properties of formate dehydrogenase from Pseudomonas aeruginosa. Characterization of haem and iron-sulphur centres by magnetic-circular-dichroism and electron-paramagnetic-resonance spectroscopy

The purification of formate dehydrogenase (FDH) from Pseudomonas aeruginosa after anaerobic growth on nitrate-containing medium was carried out. The separation of the FDH enzyme from nitrate reductase (NiR), which are found together in a particle fraction and constitute the short respiratory chain of this bacterium, has been followed by optical, magnetic c.d. (m.c.d.) and e.p.r. spectroscopy. These techniques have allowed the haem, iron-sulphur clusters and molybdenum components to be detected and, in part, their nature to be determined. Attempts to extract FDH anaerobically in the absence of sodium dithionite led to loss of activity. Addition of sodium dithionite maintained the activity of the enzyme, even after subsequent exposure to air, in an assay involving formate reduction with Nitro Blue Tetrazolium as reductant. Three preparations of FDH have been examined spectroscopically. The preparations vary in the amount of contaminating nitrate reductase, the amount of cytochrome c present and the concentration of oxidized [3Fe-4S] cluster. Optical spectra and low-temperature m.c.d. spectroscopy show the loss of a cytochrome-containing protohaem IX co-ordinated by methionine and histidine as NiR is separated from the preparation. In its purest state FDH contains one molecule of cytochrome co-ordinated by two histidine ligands in the oxidized state. This cytochrome has an e.p.r. spectrum with gz = 3.77, the band having the unusual ramp shape characteristic of highly anisotropic low-spin ferric haem. It also shows a charge-transfer band of high intensity in the m.c.d. spectrum at 1545 nm. It has recently been shown [Gadsby & Thomson (1986) FEBS Lett. 197, 253-257] that these spectroscopic properties are diagnostic of a bishistidine co-ordinated haem with steric constraint of the axial ligands. The e.p.r. and m.c.d. spectra of the reduced state of FDH reveal the presence of an iron-sulphur cluster of the [4Fe-4S]+ type. The g-values are 2.044, 1.943 and 1.903. An iron-sulphur cluster of the class [3Fe-4S], detected by e.p.r. spectroscopy in the oxidized state and by low-temperature m.c.d. spectroscopy in the reduced state, is purified away with the NiR. Finally, an e.p.r. signal at g = 2.0 with a narrow bandwidth which persists to 80 K is observed in the purest preparation of FDH. This may arise from an organic radical species.

Acid phosphatase deficient mutants of Schizosaccharomyces pombe are defective in tyrosine uptake

The uptake of tyrosine and arginine into wild type and acid phosphatase deficient mutants (pho 1) of Schizosaccharomyces pombe was investigated. All 11 pho 1-alleles tested exhibited a reduced tyrosine uptake and impaired uptake cosegregated with the lack of acid phosphatase activity. Kinetic analyses using wild type cells grown in high phosphate medium (acid phosphatase repressed) and low phosphate medium (acid phosphatase derepressed) showed staturation kinetics for tyrosine with a KM of about 2 x 10(-4) M for both media and a V of about 5 nmol min-1 mg-1 and 2 nmol min-1 mg-1 for derepressed and repressed cells respectively. The pho 1-118 strain completely lacked this saturable uptake system for tyrosine. Preliminary evidence suggests that tyrosine uptake may be via a general amino acid permease system and we conclude that mutations in the structural gene of acid phosphatase which abolish enzyme activity lead to a loss of this uptake system. In contrast to tyrosine, arginine uptake seems not to be significantly affected either by different acid phosphatase levels in wild type cells or by the pho 1-118 mutation.

A new mutation for multiple drug resistance and modified plasma membrane ATPase activity in Schizosaccharomyces pombe

The mutant JV66 was selected from the wild type strain of S. pombe 972h- ade7-413 by its ability to grow on solid rich medium containing 200 micrograms Dio-9/ml. The single nuclear mutation, designated pma1 gives resistance towards diguanidines and several other positively charged compounds. The pma1 mutation also decreases plasma membrane ATPase activity and confers resistance of ATPase to vanadate. The pma1 locus is localized on chromosome I at 5.3 map units from cyh1-C7 and at about 20.7 map units from the centromere. This new mutation is genetically and phenotypically different from the mutation cyh3 and cyh4 previously described (Johnston and Coddington 1983).

Multiple drug resistance in the fission yeast Schizosaccharomyces pombe: evidence for the existence of pleiotropic mutations affecting dependent transport systems

The uptake of L-tyrosine into wild type and antibiotic resistant strains of Schizosaccharomyces pombe requires an energy source, is initially linear with respect to time, is inhibited by 2,4-dinitrophenol and sodium azide and is saturable. However the initial uptake rates and the amount of L-tyrosine accummulated by antibiotic resistant strains are much less than wild type. Comparison of the kinetic constants of uptake shows that mutant strains have a reduced maximum velocity of uptake compared to wild type and a larger Km. Since the three mutant strains possess a permeability barrier to L-tyrosine as well as being drug resistant this is an indication that antibiotic resistance may be caused by a decrease in plasma membrane permeability.

Characterisation of ribosomes from drug resistant strains of Schizosaccharomyces pombe in a poly U directed cell free protein synthesising system

Mutants of Schizosaccharomyces pombe were isolated as resistant either to trichodermin or to anisomycin. Growth tests showed that the majority of mutants isolated were cross resistant to both drugs and also to cycloheximide. A limited genetic analysis showed that mutants at least four loci, tri3, tri4, ani1 and ani2, had this phenotype as was also the case for mutants at three cycloheximide resistant loci, cyh2, cyh3 and cyh4 reported previously (Ibrahim and Coddington, 1976). Allelism tests showed that the tri3, ani2 and cyh4 strains were allelic. A mutant at another trichodermin resistant locus, tri5, was cross resistant to anisomycin but sensitive to cycloheximide. Ribosomes from wild type and selected strains were analysed in a poly U directed cell free protein synthesising system. Three strains, cyh1-C7, ani1-F1 and tri-N15 (probably a tri5 allele) possessed ribosomes which were more resistant than the wild type to the drugs used in their isolation. In each case the site of the resistance was in the 60S subunit. Ribosomes from the cyh2, cyh3 and cyh4 strains were as sensitive to cycloheximide as those from wild type.

Genetical studies on revertants to sensitivity from a cycloheximide resistant strain of Schizosaccharomyces pombe

Six UV induced cycloheximide-sensitive revertants were isolated from the cyh1-C7 strain of Schizosaccharomyces pombe which is resistant to cycloheximide. In all cases reversion to sensitivity was due to a forward mutation in a second suppressor gene. Genetical analysis showed that at least two genes, designated scr1 and scr2 (scr=suppression of cycloheximide resistance) were involved. Both scr1 and scr2 suppressed the resistance of six independently isolated alleles at the cyh1 locus. They had no effect on two known nonsense mutations in the ade7 locus. The cyh1-C7 strain has an altered 60S ribosomal protein which can be detected by two-dimensional polyacrylamide gel electrophoresis. In two suppressed strains, cyh1-C7 scr1 and cyh1-C7 scr2, the original altered protein was present. However no further ribosomal protein differences were observed which could be correlated with the presence of the scr genes. Both scr mutations conferred cold sensitivity on the organism indicating that they were of the missense type. Hence it seems certain that scr1 and scr2 are not mutations in tRNA genes leading to either nonsense or missense suppression. There is however no direct evidence that they code for ribosomal proteins and exert their effect on cyh1-C7 at the ribosomal level.

Genetic studies on cycloheximide-resistant strains of Schizosaccharomyces pombe

Cycloheximide-resistant mutants of Schizosaccharomyces pombe were isolated either as spontaneous mutants or after mutagenic treatment with nitrous acid, UV and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Twenty-three spontaneous mutants and 64 induced mutants were analysed genetically. Crosses revealed that at least four loci, designated cyh1, cyh2, cyh3 and cyh4 are responsible for resistance. Alleles of cyh1 show good growth on either high (100 mug/ml) or low (40 mug/ml) concentrations of cycloheximide whereas alleles at the cyh2, cyh3 and cyh4 loci gorw well on 40 mug/ml but poorly on 100 mug/ml. Some alleles at the cyh2 and cyh3 loci are also temperature sensitive (ts), the ts phenotype being conferred by the same gene as the resistance. In diploids, cyh1 and cyh4 are re-essive to wild type whereas cyh2 and cyh3 are semi-dominant. There was no intragenic complementation between three cyh1 alleles. Cross-resistance to trichodermin and anisomycin was shown by cyh2, cyh3 and cyh4 but not cyh1. Most cyh1 alleles, of spontaneous and UV origin only, were cold sensitive (cs) at 14 degrees and some of these were also cycloheximide dependent at the same temperature. It is suggested that the cyh1 and cyh4 genes are involved in ribosome formation or function and the other loci probably affect the uptake of cycloheximide by the cells.

The product of the ade1: gene in Schizosaccharomyces pombe: a bifunctional enzyme catalysing two distinct steps in purine biosynthesis

The assignment of the known ade genes to steps in purine biosynthesis in Schizosaccharomyces pombe has been completed with the demonstration that an ade3 mutants lacks FGAR amidotransferase, ade1A mutants lack GAR synthetase and ade1B mutants lack AIR synthetase. A comparison of enzyme activity with map position for ade1 mutants shows that (1) complementing ade1A mutants lack GAR synthetase but posses wild type amounts of AIR synthetase, (2) complementing ade1B mutants lack AIR synthetase but posses variable amounts of GAR synthetase, (3) non-complementing mutants lack both activities. In wild type strains the two activities fractionate together throughout a hundred-fold purification. Hence the ade1 gene appears to code for a bifunctional enzyme catalysing two distinct steps in purine biosynthesis. The two activities are catalysed by two different regions of the polypeptide chain which can be altered independently by mutation. Gel filtration studies on partially purified enzymes from wild type and various complementing mutant strains, indicate that the bifunctional enzyme is a multimer consisting of between four and six sub-units of 40,000 daltons each. GAR synthetase activity is associated with both the monomeric and multimeric forms but AIR synthetase is only associated with the multimer. A comparison of enzyme levels between diploids and their original complementing haploid strains suggests that complementation is due to hybrid enzyme formation.

Biochemical studies on the nit mutants of Neurospora crassa

One allele at each of the five nit loci in Neurospora crassa together with the wild type strain have been compared on various nitrogen sources with regard to (i) their growth characteristics (ii) the level of nitrate reductase and its associated activities (reduced benzyl viologen nitrate reductase and cytochrome c reductase) (iii) the level of nitrate reductase and (iv) their ability to take up nitrite from the surrounding medium. Results are consistent with the hypothesis that nit-3 is the structural gene for nitrate reductase, nit-1 specifies in part of molybdenum containing moiety which is responsible for the nit-3 gene product dimerising to form nitrate reductase, nit-4 and nit-5 are regulator genes whose products are involved in the induction of both nitrate reductase and nitrite reductase and nit-2 codes for a generalised ammonium activated repressor protein. Studies on the induction of nitrate reductase (and its associated activities) and nitrite reductase in wild type, nit-1 and nit-3 in the presence of either nitrate or nitrite suggest that each enzyme may be regulated independently of the other and that nitrite could be true co-inducer of the assimilatory pathway. Nitrite uptake experiments with nit-2, nit-4 and nit-5 strains show that whereas nit-4 and nit-5 are freely permeable to this molecule, it is unable to enter the nit-2 mycelium.