The Two Operons of the Histidine Utilization System in Salmonella typhimurium

Abstract

Abstract We have constructed an F factor bearing the genes (hut) responsible for the synthesis of the histidine-degrading enzymes in Salmonella typhimurium. This was accomplished by transducing the genes from the chromosome onto an F factor from Escherichia coli bearing the gal-bio region, which brackets the hut genes but does not include them. Recovery of this nonhomologous transduction was made possible by deleting the homologous hut region from the chromosome. Studies of the control of enzyme synthesis in heterozygous merodiploids showed that the hut region is composed of two operons. One operon includes the structural genes for histidase and urocanase, the first two enzymes of the pathway, and a promoter-operator complex controlling their expression in cis. The second operon includes the structural genes for 4-imidazolone-5-propionate amidohydrolase and N-formimino-l-glutamate formiminohydrolase, the last two enzymes of the pathway, and a promoter gene controlling their expression in cis. Located between these two operons is a regulatory gene, whose product presumably represses both operons in trans. The target site of catabolite repression of histidase and urocanase is the promoter of their operon, as is the case in the lac operon of E. coli.