SYNTHESIS OF THE ISOLEUCYL-TRANSFER RNA AND VALYL-TRANSFER RNA-SYNTHETASES AND THE ISOLEUCINE-VALINE BIOSYNTHETIC-ENZYMES IN A THREONINE DEAMINASE REGULATORY MUTANT OF ESCHERICHIA-COLI K-12

[摘要] A mutation in the structural gene for threonine deaminase, ilvA538, results in lower than normal levels of the isoleucyl, valyl- and leucyl-tRNA synthetases. 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 vitro evidence was found 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 MW complex of the 3 molecules. A model was developed to explain the regulation of the isoleucyl- and valyl-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 valyl-tRNA synthetase, respectively. Apparently, 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.