Mutants regulatory histidine

The Jacob-Monod theory therefore predicts two types of constitutive mutants, one unlinked to the operon and recessive, and the other adjacent to the operon and cis-dominant. It generated some surprise, then, when regulatory mutants of the histidine operon were found to fall into a total of six different chromosomal positions. 

Bacteria with regulatory mutations mapping in the hisS gene have been found to have a defective histidyl-tRNA synthetase [19,98]. The finding that synthetase mutants are derepressed indicates that histidine by itself does not act in the repression mechanism, since a reduced ability of this enzyme to esterify histidine to transfer RNA should if anything cause a buildup of the free histidine pool. Instead, histidine must at least bind to histidyl-tRNA synthetase for repression to occur the enzyme-histidine complex could be the repressor, or aminoacylated tRNA could be required. Earlier work of Schlesinger and Magasanik [10] had also indicated that histidine must pass beyond the free state... 

Of all the regulatory mutants, those of hisS are the only ones eapable of being repressed by the addition of histidine to the medium [15]. This is true for some, but not all, the hisS mutants. This peculiarity would be explicable if the defect in some of the mutant histidyl-tRNA synthetases were in their affinity for histidine. Examination of the kinetic properties of a number of the mutant enzymes by De Lorenzo et al. [98] has confirmed this supposition. In mutants capable of being repressed by the addition of histidine, the synthetase has a significantly increased K for histidine, but an approximately normal for tRNA h On the other hand, in hisS mutants that do not respond to histidine, the synthetase has a higher K, for tRNA" while its histidine K is nearly normal. The occurrence of hisS mutants whose only defect appears to be in the binding of tRNA implies that simple binding of histidine to the enzyme is not sufficient to actuate repression, but that tRNA" is also involved in the repression mechanism. More positive evidence for this point comes from an analysis of hisR mutants. 

Enzyme Levels of Some Histidine Regulatory Mutants ... 

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]. 

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