Histidine operon levels

For example, the leader peptide for the phenylalanine operon includes 7 phenylalanine residues among 15 residues. The threonine operon encodes enzymes required for the synthesis of both threonine and isoleucine the leader peptide contains 8 threonine and 4 isoleucine residues in a 16-residue sequence. The leader peptide for the histidine operon includes 7 histidine residues in a row. In each case, low levels of the corresponding charged tRNA causes the ribosome to stall, trapping the nascent mRNA in a state that can form a structure that allows RNA polymerase to read through the attenuator site. 

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]. 

The tryptophan, histidine, leucine, phenylalanine, and threonine operons are regulated by attenuation. Repressors and activators also act on the promoters of some of these operons, allowing the levels of these amino acids to be very carefully and rapidly regulated. 

Represslble Biosynthetic Systems. In enteric bacteria the represslble systems for histidine and tryptophan biosynthesis provide examples of systems controlled by positive and negative elements, respectively (see Table IV). Since the level of histidine In the colon Is among the lowest of all amino acids, expression of the histidine biosynthetic operon In the bacterium will be In hl demand (Table III). Thus, demand theory predicts that the histidine biosynthetic operon will be positively regulated (Table II). In fact, there Is good evidence to show that this operon Is controlled primarily by an antiterminator mechanism (, ). On the other hand, the level of tryptophan In the colon Is among the hipest of the amino acids and, therefore, expressl( i of the tryptophan biosynthetic operon In the bacterium... 

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). 

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... 

---