Operator locus

A Gene Was Discovered That Leads to Repression of Synthesis in the Absence of Inducer A Locus Adjacent to the Operon Is Found to Be Required for Repressor Action Genetic Studies on the Repressor Gene and the Operator Locus Lead to a Model for Repressor Action... 

Genetic Studies on the Repressor Gene and the Operator Locus Lead to a Model for Repressor Action... 

The behavior of the various mutations we have just discussed led Jacob and Monod to propose a model for the regulation of protein synthesis. The genetic elements of this model consist of a structural gene or genes, a regulator gene, and an operator locus (fig. 30.6). 

The regulator gene (not itself part of the operon) produces a repressor that can interact with the operator locus. 

The repressor molecule combines with the operator locus to prevent the structural gene(s) from synthesizing mRNA. In induction, the inducer combines with the repressor, changing its structure so that it no longer binds to the operator this region of the genome is then free to combine with RNA polymerase. 

Schematic model illustrating the operon hypothesis. This diagram is modified from the original proposed by Jacob and Monod, who thought i gene repressor was an RNA rather than a protein, (a) The i gene encodes a repressor that binds tightly to the operator o locus, thereby preventing transcription of the mRNA from the z, y, and a structural genes. (b) When inducer is present, it combines with repressor, changing its structure so it can no longer bind to the operator locus. Inducer also can remove repressor already complexed with the o locus.

The operator locus (the presumptive repressor binding site). Bases are numbered +1 for the first base transcribed and — 1 for the base before that. Regions showing dyad symmetry are underlined and overlined. Arrows indicate point mutations leading to the constitutive phenotype (oc). Circled bases are those groups that are strongly... 

Schematic diagram of the repressor control of trp operon expression. The trp promoter (P) and trp operator (O) regions overlap. The trp aporepressor is encoded by a distantly located trpR gene. L-Tryptophan binding converts the aporepressor to the repressor that binds at the operator locus. This complex prevents the formation of the polymerase-promoter complex and transcription of the operon that begins in the leader region (trpL). Only a fraction of the transcripts extends beyond the attenuator locus in the leader region. The regulation of this fraction is discussed in the text.

The controlling elements of the operon consist of a promoter locus (j>), an operator locus (o), and a regulator gene (/). These will be introduced here and discussed in greater detail in Sections 111 and IV. 

In any hypothesis concerning the nature of the operator, its specific complementary configuration must be genetically determined therefore the operator could be affected by mutations which would alter or abolish its specific affinity for the repressor. Such mutations might result in constitutive synthesis of the protein or proteins if no other functions were impaired. These mutations should define an operator locus the most distinctive predictable property of operator mutants is that the constitutive allele should be dominant to the wild type, since the repressor-sensitive operators should not prevent the functioning of repressor-insensitive operators. 

Dyke (personal communication, 1970) has obtained a cis-dominant constitutive mutant which shows o" character, and there may be functional equivalence in the penicillinase and 8-galactosidase systems of the operator locus. 

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