title: Synthesis of the isoleucyl- and valyl-tRNA synthetases and the isoleucine-valine biosynthetic enzymes in a threonine deaminase regulatory mutant of Escherichia coli K-12.
abstract: A mutation in the structural gene for threonine deaminase, ilvA538 , results in lower than normal levels of the isoleucyl, valyl- and leucyl-tRNA synthetases. Moreover, this regulatory mutation decreases the level of expression of the ilv biosynthetic operons and renders their expression non-responsive to limitations of the branched-chain amino acids. In this paper, we present in vitro evidence for the inhibition of isoleucyl- and valyl-tRNA synthetase activity by threonine deaminase and 2-ketobutyrate, the product of the threonine deaminase reaction, through the formation of a high molecular weight complex of the three molecules. Based on these results, we propose a model to explain the regulation of the isoleucyl- and valyt -tRNA synthetases in which transient inhibition of the synthetase enzyme activities by threonine deaminase and 2-ketobutyrate increases the expression of ileS and valS , the structural genes for isoleucyl- and valyt -tRNA synthetase, respectively. Further, the results suggest that the hyperattenuated expression of the ilv biosynthetic operons is due to an increased rate of complex formation of valyl and isoleucyl-tRNA synthetases and the altered form of threonine deaminase of the ilvA538 mutant strain.
mesh_terms: D000604:Amino Acyl-tRNA Synthetases / Q000096:biosynthesis*; D002087:Butyrates / Q000494:pharmacology; D002850:Chromatography, Gel; D004926:Escherichia coli / Q000235:genetics*; D007533:Isoleucine-tRNA Ligase / Q000096:biosynthesis*; D009154:Mutation; D013913:Threonine Dehydratase / Q000235:genetics* / Q000378:metabolism; D014633:Valine / Q000378:metabolism; D014637:Valine-tRNA Ligase / Q000096:biosynthesis*
chemical_list: D002087:Butyrates; C005087:alpha-ketobutyric acid; D013913:Threonine Dehydratase; D000604:Amino Acyl-tRNA Synthetases; D007533:Isoleucine-tRNA Ligase; D014637:Valine-tRNA Ligase; D014633:Valine