Histidine operon repression

Responses of the regulatory mutants to the composition of the growth medium are also of interest. Like the wild type, nearly all the mutants do not reduce their enzyme levels when histidine is added to minimal medium. The only exceptions are hisS mutants having a histidyl-tRNA synthetase with a poorer K for histidine (see Section IV,E). Addition of all the amino acids, or growth on nutrient broth, however, does result in a lowering of the enzyme levels. In the case of hisO, hisR, and hisT, the reduction in enzyme levels might not reflect a specific repression of the histidine operon, but instead be due to the... 

Study of the repression control of the histidine operon has suggested that it may be more complicated than the model proposed by Jacob and Monod. This possibility is predicated on failure to find a pure repressor gene. This failure has left open the possibility that control may be exerted by aminoacylated tRNA alone, possibly at the level of protein synthesis rather than mRNA synthesis. Alternatively, the repressor could be encoded by one of the regulatory genes discussed, but could also serve an additional function vital to the cell. The necessity of maintaining this second function would prevent the isolation of mutants which had totally lost repressor activity. In line with this latter possibility, the complex of histidyl-tRNA synthetase with aminoacylated tRNA may serve as the repressor. The mutual affinity of these two macromolecules and their concentrations within the cell are such that a large portion of the aminoacylated tRNA may be complexed to the synthetase [20,118a]. 

Goldberger and Berberich (1965) carried out some interesting work to determine the time sequence of activity of the cistrons of the histidine operon based on the kinetics of repression and derepression of five (of the ten) enzymes of this operon from the beginning, middle, and end regions of the histidine operon of S. typhimurium. The specific activity of these enzymes was measured during a period soon before and after removal of histidine from the growth medium of the "leaky" mutant of this microorganism. 

Another control over termination that has been extensively studied is the attenuation of transcription at the end of the leader regions of several amino acid biosynthetic operons (or genes) in E. coVl and related organisms (39). The primary indication that such a mechanism is involved in the control of an amino acid biosynthetic pathway is an endproduct repression that is dependent on the amino acid being transferred to its cognate tRNA at an ample rate. Thus, derepression of the histidine biosynthetic pathway can be achieved by any mechanism that reduces the intracellular level of histidyl tRNA, for example, by limiting the supply of histidine itself or by limiting the activity of the histidyl tRNA synthetase (51). 

As in the case for derepression, two kinetic patterns for repression of the histidine enzymes have been observed by Goldberger and his colleagues [79]. Unlike the situation for derepression, however, the mode of repression is not related to the formylating capacity of the cell. Instead, the pattern of repression depends on the condition of the feedback site of the first enzyme in histidine biosynthesis, the PR-ATP synthetase [79,80]. When the feedback site is able to function properly, repression is sequential, with the time of repression for eaeh enzyme reflecting the position of its gene in the operon. When the affinity of the feedback site of the PR-ATP synthetase for histidine is reduced, either by mutation or by the addition of a histidine analog which binds to the feedback site, synthesis of all the histidine enzymes ceases simultaneously [79]. 

The finding that a decrease in the level of tRNA" in the cell causes derepression of the operon complements the data from studies of hisS mutants tRNA" is certainly involved in the repression mechanism. However, in the cell tRNA" exists in two forms— one acylated with histidine and the other free. Either of these species could act in the repression process. Since the pool size of acylated tRNA" reflects the... 

This review has pointed out that the major control element in histidine production is feedback inhibition of the first enzyme of the pathway. However, most of the work on regulation of the operon has been devoted to repression control. This is partly because less is known about... 

Studies of mutation in the histidine pathway of 5. typhimurium have revealed another interesting relationship between genes and operon—coordinate repression. When the wild type of S. typhimurium is grown in a medium containing histidine, the activities of the enzymes involved in histidine biosynthesis are depressed. The activities of all enzymes are depressed equally so that in the mutant, the ratio of the activity of one enzyme to that of another enzyme is the same as in the wild type. To explain coordinated repression it is proposed that the entire base sequence in the operon is translated into a single molecule of messenger RNA. 

Negative repression was investigated in the tryptophan (trp) synthesis operons of E. coli and Salmonella typhimurium, and the histidine synthesis operon of Salmonella. The trp operon is totally derepressed in the presence of nonsurplus amounts of tryptophan. Here, transcription and translation go at maximal rates. Enzymes of the tryptophan system synthesized it in amounts which are totally utilized during translation. This condition is known as turn on. The activity of the first enzyme in the sequence of tryptophan synthesis — anthranilate synthetase — is inhibited within several seconds after the addition of surplus tryptophan. The aporepressor becomes the repressor and inhibits transcription of the operon (negative repression) after it associates with the effector, tryptophan. This association occurs if the surplus of tryptophan exists for a sufficient amount of time. After several minutes, most of the resulting mRNA is degraded and the rate of synthesis is noticeably reduced (known as the turn of condition). 

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