Genetics / D.J. Cove.

Cove, D. J.

Date:: 1971

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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Genetics / D.J. Cove. Source: Wellcome Collection.

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162 The control of gene activity I : Returning to the question of the chemical nature of the repressor, this has now been established as protein. This had seemed likely as it had been known for example that it was possible to obtain amber- type i~ mutations. In 1966, W. Gilbert and B. Müller-Hill reported that they had been able to purify the lactose system repressor, using the technique of equilibrium dialysis. The repressor is a protein of co-inducer A iwod^uct ^^ repressor ^-galactosidase prevents gene activity I о z regulator operator structural gene gene gene Figure 78. Model to explain the inducibility of jS-galactosidase activity, ^-galactosidase synthesis can occur for three reasons : 1. The repressor is inactivated by the co-inducer. 2. A mutation in the i gene prevents the production of an active repressor. 3. A mutation in the о gene prevents the recognition of the z gene by the repressor. about 150 ООО molecular weight, which binds tightly to co-inducers, such as IPTG. The purification of the i gene product also enabled the other question, that of the level of gene activity at which regulation was operative, to be answered. Assays for the lactose operon mRNA which had been devised, had shown earlier that not only did the level of jS-galactosidase increase on induction, but also the level of mRNA from the operon. This cannot be taken as rigorous proof that trans¬ cription is the primary target of the control system, as there might be some feedback system operative which prevented transcription, if the mRNA produced was not translated. However it has now been shown that purified i gene product has an affinity for the DNA of the operator region, provided co-inducer is not present, but no affinity for the corresponding mRNA, or for denatured single stranded DNA. Control in the lactose system therefore appears to be at the trans¬ criptional level.