Histidine operon deattenuation in dnA mutants of Salmonella typhimurium correlates with a decrease in the gene dosage ratio between tRNA(His) and histidine biosynthetic loci

Expression. of the histidine operon of Salmonella typhimurium is increased in dnaA(Ts) mutants at 37 degrees C. This effect requires an intact his att enuator and can be suppressed by increasing the gene copy number of the his R locus, which encodes the tRNA(HIS). We present data which suggest that th e his deattenuation defect in dnaA(Ts) mutants results from the loss of a g ene dosage gradient between the hisR locus, close to oriC, and the his oper on, far from oriC. Some of the conclusions drawn here may apply to other op erons as well.