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  • Article
    White RH.
    Biochim Biophys Acta. 2003 Dec 05;1624(1-3):46-53.
    The pathway for the biosynthesis of cysteine and homocysteine in Methanococcus jannaschii has been examined using a gas chromatography-mass spectrometry (GC-MS) stable isotope dilution method to identify and quantitate the intermediates in the pathways. The first step in the pathway, and the one responsible for incorporation of sulfur into both cysteine and methionine, is the reaction between O-phosphohomoserine and a presently unidentified sulfur source present in cell extracts, to produce L-homocysteine. This sulfur source was shown not to be sulfide. The resulting L-homocysteine then reacts with O-phosphoserine to form L-cystathionine, which is cleaved to L-cysteine. The pathway has elements of both the plant and mammalian pathways in that the sulfur is first incorporated into homocysteine using O-phosphohomoserine as the acceptor and the resulting homocysteine, via transsulfuration, supplies the sulfur for cysteine formation. The pathway leading to these two amino acids represents an example of metabolic thrift where the preexisting cellular metabolites O-phosphohomoserine and O-phosphoserine are used as the ultimate source of the carbon framework for the biosynthesis of these amino acids. These findings explain the absence of identifiable genes in the genome of this organism for the biosynthesis of cysteine and homocysteine.
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  • Article
    Wu RF, Osatomi K, Terada LS, Uyeda K.
    Biochim Biophys Acta. 2003 Dec 05;1624(1-3):29-35.
    Previously, we found a novel protein factor in the livers of rats fed a high-carbohydrate diet, which binds to the major late transcription factor (MLTF)-like site within the glucose response element (GRE) of the liver-type pyruvate kinase (L-PK) gene [J. Biol. Chem. 274 (1999) 1100]. This factor, termed glucose response element binding protein (GRBP), exists in both liver cytosol and nucleus. In order to identify GRBP, we purified to homogeneity cytosolic GRBP from rat liver extract and identified it as a Translin/Trax heteromeric complex. Based on partial amino acid sequences, we have cloned full-length rat cDNAs of both Translin and Trax. The nuclear and the cytosolic Translin/Trax complex were both large polymers of 240 and 420 kDa, respectively. The molar ratio of Translin/Trax in the polymers was 2:1 in the liver cytosols. The nuclear and cytosolic Translin/Trax complexes as well as expressed His-tagged Translin bound to double- and single-stranded MLTF sites of the GRE of L-PK gene more avidly than to single-stranded Bcl-CL1, which was initially thought to be specific for Translin. Our findings indicate that the Translin/Trax complex constitutes the previously described GRBP, and that this complex binds the GRE of the L-PK gene with high affinity. The precise physiologic role of GRBP, however, remains unclear.
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