P

lactose appears to be the inducer here, allolactose is actually responsible for induction.

In the lac operon, the lacZ, lacY, and lacA genes have a common promoter (lacP in Figure 16.7a) and are transcribed together. Upstream of the promoter is a regulator gene, lacI, which has its own promoter (PI). The lacI gene is transcribed into a short mRNA that is translated into a repressor. Each repressor consists of four identical polypeptides and has two binding sites; one site binds to allolactose and the other binds to DNA. In the absence of lactose (and, therefore, allolactose), the repressor binds to the lac operator site lacO (see Figure 16.7a). Jacob and Monod mapped the operator to a position adjacent to the lacZ gene; more recent nucleotide sequencing has demonstrated that the operator actually overlaps the 3' end of the promoter and the 5' end of lacZ (< Figure 16.8).

Immediately upstream of the structural genes is the lac promoter. RNA polymerase binds to the promoter and moves down the DNA molecule, transcribing the structural genes. When the repressor is bound to the operator, the binding of RNA polymerase is blocked, and transcription is prevented. When lactose is present, some of it is converted into allolac-tose, which binds to the repressor and causes the repressor to be released from the DNA. In the presence of lactose, then, the repressor is inactivated, the binding of RNA polymerase is no longer blocked, the transcription of lacZ, lacY, and lacA takes place, and the lac enzymes are produced.

Have you spotted the flaw in the explanation just given for the induction of the lac enzymes? You might recall that permease is required to transport lactose into the cell. If the lac operon is repressed and no permease is being produced, how does lactose get into the cell to inactivate the repressor and turn on transcription? Furthermore, the inducer is actually allolactose, which must be produced from lactose by p-galactosidase. If p-galactosidase production is repressed, how can lactose metabolism be induced?

lacZ gene

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