Histidine Regulation in Salmonella typhimurium

The histidyl-tRNA synthetase has been partially purified from five hisS mutants of Salmonella typhimurium and the kinetic constants compared to those previously obtained with the wild type enzyme. The mutants were selected on the basis of being derepressed for the histidine operon. Some of the mutant enzymes have markedly decreased affinities for histidine (one has a 36-fold decrease in affinity) and this has been correlated with the physiological behavior of the mutants. One of the mutant enzymes, that of hisS1587, has a markedly increased affinity for histidine and ATP and a 7-fold decreased affinity for tRNAhis. Another mutant enzyme, that of hisS2280, has normal affinities for histidine and for tRNAhis but a 5-fold decreased affinity for ATP. Analysis of the mutants has not helped to answer the question of whether the derepression of the histidine operon caused by altered synthetases is due solely to the decreased amount of aminoacylated tRNAhis in the cell, or whether the synthetase is also involved as a repressor in complex with charged tRNAhis.