Lac mRNA synthesis

Thus, the overall regulatory pattern is as follows A bacterium growing in a medium without lactose does not make either the lac z or lac y product because the repressor that is made is bound to the operator and prevents synthesis of lac mRNA. The cell grows by utilizing whatever other carbon source is available. If lactose is added (and if glucose is absent see below), basal level lac y and lac z proteins bring the lactose into the cell and convert it to allolactose. As a result, the repressor is inactivated, RNA polymerase binds to the promoter, and synthesis of lac mRNA begins. Lac mRNA synthesis continues until the lactose is exhausted, in which case the inactive repressor would be reactivated and repression reestablished. 

Crombrugghe, B. de, Chen, B., Gottesman, M., Pastan, I., Varmus, H. E., Emmer, M., Perlman, R. L. Regulation of lac mRNA synthesis in a soluble cell-free system. Nature (Lond.) New Biol. 230, 37-40 (1971a). 

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