Metabolit-induzierte inaktivierung von glutaminsynthetase aus Escherichia coli im zellfreien system

Biosynthesis of the branched-chain amino acids in yeast: a leucine-binding component and regulation of leucine uptake

Use of an ion-exchange resin assay has shown that leucine is bound to a component of a dialyzed extract of yeast. Leucine binding may be related to in vivo uptake of the amino acid. A yeast strain with a 30-fold lower affinity for leucine uptake in vivo has a parallel reduction in affinity for in vitro leucine binding; the rate of leucine uptake in wild-type yeast can be increased four- to fivefold by growth on leucine as a sole nitrogen source. Under these conditions, the specific activity of the leucine-binding component also increases over threefold. Regulation of leucine uptake was studied by using wild-type strain 60615 and a mutant 60615/fl(2) with a constitutively elevated leucine uptake system. Leucine pool formation in the mutant was accompanied by an overshoot, leading to a loss of leucine from the pool. The phenomenon could be observed in the wild type under certain conditions. The mechanism of this process was examined. The leucine uptake system was found to be stable in the absence of protein synthesis. The rate of leucine uptake increased on reduction of the pool of amino acids, and in strain 60615/fl(2) the ability to overshoot was rapidly recovered on depletion of the leucine pool. The results suggest a control of leucine uptake by feedback inhibition, in which leucine or other amino acids, e.g., isoleucine, inhibit leucine uptake. The results do not exclude control by a rapidly activated-inactivated system.

Regulation of synthesis of glutamine synthase in Bacillus subtilis

A study of the regulation of the synthesis of the enzyme glutamine synthase in Bacillus subtilis was initiated. An assay, based on the measurement of glutamo-hydroxamate, was used to characterize the enzyme in crude preparations and in toluene-treated cells. Determinations were made of the Michaelis constants for adenosine triphosphate, hydroxylamine, and glutamate (9 x 10(-3), 4 x 10(-3), and 2.2 x 10(-2)m, respectively), the pH optimum (7.6 to 7.7), and the stability. The differential rate of synthesis was determined under various growth conditions. The enzyme was found to be relatively insensitive to regulation. Partial repression was caused by glutamine, arginine, asparagine, and glutamate, or by carbon limitation in a chemostat. Derepression was caused by exhaustion of externally added amino acids or by nitrogen limitation in a chemostat.