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.-In several instances, the rate of synthesis of the enzymes of a biosynthetic pathway appears to be regulated by the size of the pool of the end product of that pathway.1-4 This phenomenon has been called enzyme repression.2 We have observed repression by histidine during the investigation of the histidine biosynthetic enzymes in histidine-requiring mutants of Salmonella typhimurium.6 The cellular concentration of the four histidine biosynthetic enzymes which were examined were increased about 15-fold by limiting the amount of histidine available to a mutant during growth. A simple method for limiting the availability of histidine in the cell during the entire growth period, and thus increasing the enzyme concentrations, was realized by growing histidine-requiring mutants on a derivative of histidine, formylhistidine, as a source of this amino acid. It appeared, therefore, that the normal low level of the biosynthetic enzymes in wild-type cells grown on minimal medium was probably due to the repression by the normal pool of histidine. of most of the cellular capacity for synthesizing these proteins. This investigation is concerned with utilizing the histidine biosynthetic system from Salmonella to answer the following question: Does the histidine repression of enzyme synthesis affect each of the enzymes of the pathway to the same extent, i.e., is there a parallel increase in specific activity of all four enzymes on lowering the internal histidine pool and a constant ratio of the activity of one histidine enzyme to another independent of the concentration of the enzymes in the cell? A secondary point of interest is whether there is any influence of the concentration of the enzyme substrates on the enzyme synthesis control mechanism. The strains used in this study were some of the several hundred histidine-requiring' mutants of Salmonella that have been mapped by Hartman, et al.6' 7 They were able to demonstrate that each of the mutants could be classified into one of the seven genetic loci, E, F, A, B, C, D, and G, which were closely linked on the chromosome in the above sequence. These same mutants were analyzed by us for presence or absence of various enzymes of the histidine biosynthetic pathway., The genetic data taken in conjunction with the biochemical analysis gave more direct support for Hartman's original suggestion8 that the sequence of the genes in the linkage map corresponds to the sequence of the enzymes they control in the biosynthetic pathway.

COORDINATE REPRESSION OF THE SYNTHESIS OF FOUR HISTIDINE BIOSYNTHETIC ENZYMES BY HISTIDINE