Biomedical subjects

R Fall

Publications and source records attributed to R Fall.

42 records · Page 3Linked to original sources

PubMed1986

Release of cell-free ice nuclei by Erwinia herbicola.

Several ice-nucleating bacterial strains, including Erwinia herbicola, Pseudomonas fluorescens, and Pseudomonas syringae isolates, were examined for their ability to shed ice nuclei into the growth medium. Only E. herbicola isolates shed cell-free ice nuclei active at -2 to -10 degrees C. These cell-free nuclei exhibited a freezing spectrum similar to that of ice nuclei found on whole cells, both above and below -5 degrees C. Partially purified cell-free nuclei were examined by density gradient centrifugation, chemical and enzymatic probes, and electron microscopy. Ice-nucleating activity in these cell-free preparations was associated with outer membrane vesicles shed by cells and was sensitive to protein-modifying reagents.

Bacterial Proteins↗

PubMed1985

Microbial methylation of benzenethiols and release of methylthiobenzenes.

Three phylogenetically diverse microorganisms methylated several different chloro- and nitro-substituted benzenethiols to yield the corresponding methylthiobenzenes. These products were identified by gas chromatography-mass spectrometry. In several cases large percentages of the methylthio products were released by intact cells into the medium, suggesting that microbial methylation of xenobiotic thiols may be a significant biotransformation in many ecosystems.

Ascomycota↗

PubMed1985

Characterization of a Selenium-Independent Glutathione Peroxidase From Euglena gracilis.

Light or dark grown Euglena gracilis strains contain similar levels of glutathione (GSH) peroxidase. Cells in midstationary phase of growth contained the highest level of the enzyme. The enzyme was purified 280-fold to homogeneity from the permanently bleached strain, E. gracilis var bacillaris W(3)BUL. The native enzyme has a molecular weight of 130,000 as measured by gel permeation chromatography, and contains four subunits (mol wt 31,500) as measured by sodium dodecyl sulfate gel electrophoresis. A variable amount of a higher molecular weight form of the enzyme (approximate mol wt 250,000) was detected but not further characterized. The enzyme has an isoelectric point of 4.7. No selenium could be detected in the purified enzyme. The enzyme is active with H(2)O(2) and a variety of organic hydroperoxides, including 13-hydroperoxylinoleic acid, and is specific for GSH as the thiol substrate. Apparent K(m) values for H(2)O(2), t-butyl hydroperoxide, and GSH were 0.03, 1.5, and 0.7 millimolar, respectively. A comparison of selenium-dependent and selenium-independent GSH peroxidases from various eukaryotic sources is presented.

Journal Article↗

PubMed1985

S-Adenosylhomocysteine hydrolase deficiency in cysteine auxotrophs of Tetrahymena thermophila.

The biochemical lesion in two cysteine auxotrophs of Tetrahymena thermophila has been established as a defect in S-adenosylhomocysteine hydrolase, an enzyme of the transsulfuration pathway. As a result, these mutants require cysteine (or cystathionine or homocysteine) for growth in a defined medium. Cell-free extracts of the mutants contained less than 5% of the level of the enzyme seen in the wild type. One of the mutant strains accumulated intracellular levels of S-adenosylhomocysteine as high as 1380 microM, a level 200 times normal. When both mutant strains were maintained in defined medium without cysteine, growth occurred after a long lag; this phenomenon was termed "adaptation." Adaptation was a) reversed by passage through rich medium, b) was not a recovery of S-adenosylhomocysteine hydrolase, and c) was probably linked to induction of an alternate pathway for cysteine biosynthesis, involving a lysosomal S-adenosylhomocysteine nucleosidase activity.

Adenosylhomocysteinase↗

PubMed1984

Bioconversion of xylan to triglycerides by oil-rich yeasts.

A series of lipid-accumulating yeasts was examined for their potential to saccharify xylan and accumulate triglyceride. Of the genera tested, including Candida, Cryptococcus, Lipomyces, Rhodosporidium, Rhodotorula, and Trichosporon, only Cryptococcus and Trichosporon isolates saccharified xylan. All of the strains could assimilate xylose and accumuate triglyceride under nitrogen-limiting conditions. Strains of Cryptococcus albidus were found to be especially useful for a one-step saccharification of xylan coupled to triglyceride synthesis. Cryptococcus terricolus, a strain constitutive for lipid accumulation, lacked extracellular xylanase, but did assimilate xylose and xylobiose and was able to continuously convert xylan to triglyceride if the culture medium was supplemented with xylanase.

Journal Article↗

PubMedDate not supplied

Induction of poplar leaf nitrate reductase: a test of extrachloroplastic control of isoprene emission rate.

Several recent studies have suggested that control of isoprene emission rate is in part exerted by supply of extrachloroplastic phosphoenolpyruvate to the chloroplast. To test this hypothesis, we altered PEP supply by differential induction of cytosolic nitrate reductase (NR) and PEP carboxylase (PEPC) in plants of Populus deltoides grown with NO3- or NH4+ as the sole nitrogen source. Growth with 8 mM NH4+ produced a high leaf nitrogen concentration, compared with 8 mM NO3-, as well as slightly elevated rates of photosynthesis and significantly enhanced rates of isoprene emission and content of dimethylallyl diphosphate (DMAPP, a precursor to isoprene biosynthesis), chlorophyll (a+b) and carotenoids. Growth with 8 mM NO3- resulted in parallel reductions in both leaf isoprene emission rate and DMAPP. The differential effects of growth with NH4+ or NO3- were not observed when plants were grown with 4 mM nitrogen. The effects of reduced DMAPP availability were specific to isoprene emission and were not propagated to higher isoprenoids, as the correlations between nitrogen content and either leaf chlorophyll (a+b) or total carotenoids were unaffected by nitrogen source. Biochemical analysis revealed significantly higher levels of NR and PEPC activity in leaves of 8 mM NO3- -grown plants, consistent with their fundamental roles in nitrate assimilation. Taken together, these results support the hypothesis that foliar assimilation of NO3- reduces isoprene emission rate by competing for carbon skeletons (mediated by PEPC) within the cytosol and possibly reductant within the chloroplast. Cytosolic competition for PEP is a major regulator of chloroplast DMAPP supply, and we offer a new "safety valve" hypothesis to explain why plants emit isoprene.

Butadienes↗