Molecular cloning and characterization of a plant serine acetyltransferaseplaying a regulatory role in cysteine biosynthesis from watermelon.
J Biol Chem. 1995; 270: 16321-6
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Serine acetyltransferase (SATase; EC 2.3.1.30), which catalyzes thereaction connecting serine and cysteine/methionine metabolism, plays aregulatory role in cysteine biosynthesis in plants. We have isolated acDNA clone encoding SATase by direct genetic complementation of a Cys-mutation in Escherichia coli using an expression library of Citrullusvulgaris (watermelon) cDNA. The cDNA encodes a polypeptide of 294 aminoacids (31,536 Da) exhibiting 51% homology with that of E. coli SATase.DNA-blot analysis indicated the presence of a single copy of the SATasegene (sat) in watermelon. RNA hybridization analysis suggested therelatively ubiquitous and preferential expression in the hypocotyls ofetiolated seedlings. Immunoblot analysis indicated the accumulation ofSATase predominantly in etiolated plants. L-Cysteine, an end product ofthe cysteine biosynthetic pathway, inhibited the SATase in an allostericmanner, indicating the regulatory function of SATase in this metabolicpathway, whereas beta-(pyrazole-1-yl)-L-alanine, a secondary metaboliteformed partly through the cysteine biosynthetic pathway, showed noinhibitory effect. A multi-enzyme complex was formed from recombinantproteins of SATase and cysteine synthase (O-acetylserine(thiol)-lyase)from watermelon, suggesting efficient metabolic channeling from serine tocysteine, preventing the diffusion of intermediary O-acetyl-L-serine.
L-cysteine biosynthesis in Escherichia coli: nucleotide sequence andexpression of the serine acetyltransferase (cysE) gene from the wild-typeand a cysteine-excreting mutant.
J Gen Microbiol. 1987; 133: 515-25
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Serine acetyltransferase (SAT) from Escherichia coli is subject tofeedback inhibition by L-cysteine. A mutant was isolated which excretesL-cysteine because of a lesion in cysE, the structural gene for SAT,rendering the enzyme less feedback sensitive. To analyse the structuralbasis for this mutation the cysE genes both from wild-type E. coli and themutant strain were cloned and their nucleotide sequences determined. ThecysE gene contained an open reading frame consisting of 819 bp, equivalentto a protein of 273 amino acids. The mutant gene showed a single basechange in position 767 resulting in a methionine to isoleucinesubstitution. A causal connection between this SAT sequence alteration,feedback insensitivity and L-cysteine excretion was demonstrated. The SATfrom the wild-type strain was purified. It was composed of a singlepolypeptide chain migrating in SDS gels according to an Mr of 34,000. Asin Salmonella typhimurium, the enzyme was associated in a bifunctionalcomplex with O-acetylserine (thiol)-lyase.
