Studies on the mechanism of repression of arginine biosynthesis in Escherichia coli: III. Repression of enzymes of arginine biosynthesis in arginyl-tRNA synthetase mutants

Abstract

Canavanine-resistant mutants of Escherichia coli K12 have been isolated which produce a defective arginyl-tRNA synthetase, the activity of the enzyme in various mutants ranging from about 3 to 50% of that in the wild type. On the basis of their growth rates in the presence and absence of arginine, the mutants can be put into two groups. Class I mutants (slow growth in the absence of arginine, normal growth in the presence of arginine) have an altered arginyl-tRNA synthetase with a decreased affinity for arginine. No marked changes were noticed in the enzyme obtained from class II mutants (slow growth in the absence or presence of arginine). Repression by arginine of the enzymes of arginine synthesis, as measured by the rate of formation of ornithine transcarbamylase and argininosuccinase, was not impaired in either class. Levels of charged tRNAArg in growing cells showed no correlation between the state of repression and the degree of charging. Thus for each strain tested there was no appreciable difference in the level of charging between repressed cells, growing with arginine, and de-repressed cells, growing without arginine. For class II mutants this level was about 20% of the total arginine acceptor capacity of tRNA, for class I mutants about 75%, and for the parent strains with a normal arginyl-tRNA synthetase, 80 to 90%. We conclude that the bulk of tRNAArg is not involved in repression, but we have not ruled out participation of a small fraction of this tRNA.