Previous studies showed that when triazolalanine was added to a derepressed culture of a histidine auxotroph, repression of the histidine operon occurred as though histidine had been added (6). However, when triazolalanine was added to a derepressed culture of a strain with a mutation in the first gene of the histidine operon which rendered the first enzyme for histidine biosynthesis resistant to inhibition by histidine, repression did not occur. The studies reported here represent a cis/trans test of this effect of mutations to feedback resistance. Using specially constructed merodiploid strains, we were able to show that the wild-type allele is dominant to the mutant (feedback resistant) allele and that the effect operates in trans . We conclude that the enzyme encoded by the first gene of the histidine operon exerts its regulatory effect on the operon not by acting locally at its site of synthesis, but by acting as a freely diffusible protein.

An in vitro system was developed for transcription of the histidine operon of Esherichia coli carried in the genome of a defective 480 transducing phage.The messenger RNA (mRNA) of the histidine operon synthesized in the in vitro system was detected by hybridization to single strands of both 480 and q580dhis DNA, and by competition of this hybridization with unlabeled histidine mRNA that had been synthesized in vivo (RNA extracted from cells in which the histidine operon had been derepressed

Studies were done to examine direct binding of the first enzyme of the histidine biosynthetic pathway (phosphoribosyltransferase) to 32P-labeled phi80dhis DNA and competition of this binding by unlabeled homologous DNA and by various preparations of unlabeled heterologous DNA, including that from a defective phi80 bacteriophage carrying the histidine operon with a deletion of part of its operator region. Our findings show that phosphoribosyltransferase binds specifically to site in or near the regulatory region of the histidine operon. The stability of the complex formed by interaction of the enzyme with the DNA was markedly decreased by the substrates of the enzyme and was slightly increased by the allosteric inhibitor, histidine. These findings are consistent with previous data that indicate that phosphoribosyltransferase plays a role in regulating expression of the histidine operon.