Histidine operon isolation

In an attempt to study natural polarity, mutations in different parts of the histidine operon were isolated which cause polarity, but do not result in a histidine requirement [70]. The very existence of these modulation mutants shows that it is possible for the cell to optimize the gradient of production of the enzymes of an operon. The analysis of one of these mutants is discussed in the following section. 

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]. 

Corroler, D., Desmasures, N., Gueguen, M. (1998). Correlation between polymerase chain reaction analysis of the histidine biosynthesis operon, randomly amphfied polymorphic DNA analysis and phenotypic characterization of dairy Lactococcus isolates. Applied and Environmental Microbiology, 67, 2011-2020. 

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