Regulation of glutamine synthetase. II. Patterns of feedback inhibition in microorganisms

Abstract

The feedback inhibition of glutamine synthetase was investigated by use of partially purified enzyme preparations from Salmonella typhimurium, Micrococcus sodonensis, Pseudomonas fluorescens, Bacillus cereus, Bacillus licheniformis, Clostridium pasteurianum, Rhodospirillum rubrum, Neurospora crassa, Candida utilis, and Chlorella pyrenoidosa. Inhibition analyses indicated that the enzyme of each organism can be effectively regulated with mixtures of end products from the diverse pathways of glutamine metabolism. When tested individually, tryptophan, histidine, alanine, glycine, glutamine, 5'-adenylate (AMP), cytidine-5'-triphosphate, carbamyl phosphate, and glucosamine-6-phosphate gave limited inhibition. In most cases, the inhibitors were independent in their action, and cumulative degrees of inhibition were obtained with mixtures of these end products. In contrast, with the glutamine synthetases of the two Bacillus species, the simultaneous presence of AMP and histidine (or AMP and glutamine) gave inhibition greater than the sum of the amounts of inhibition caused by either inhibitor alone. Also, alanine and carbamyl phosphate acted synergistically to inhibit the enzyme from N. crassa. The remarkable similarity in the overall patterns of end-product inhibition observed with the enzymes from different sources indicates that these diverse organisms have evolved comparable mechanisms for the regulation of glutamine metabolism. Nevertheless, the enzymes from different sources do differ significantly in their physical and catalytic properties, as was demonstrated by dissimilarities in their purification behaviors, specificity for nucleotide substrate, ability to catalyze the glutamyl transfer reaction, and ability to utilize Mn(++) and Mg(++) as activators for the biosynthetic reaction.