Rna

lycistr

Translation ors/t

Protein Protein Protein Protein A B C D

FIGURE 9.11 Components of the lac Operon

The lac operon consists of three structural genes, lacZYA, all transcribed from a single promoter, designated lacP. The promoter is regulated by binding of the repressor at the operator, lacO, and of Crp protein at the Crp site. Note that in reality the operator partly overlaps both the promoter and the structural gene. The single lac mRNA is translated to produce the LacZ, LacY, and LacA proteins. The lacI gene that encodes the LacI repressor has its own promoter and is transcribed in the direction opposite to the lacZYA operon.

The lac operon includes genes for lactose uptake and metabolism.

lation refers to regulation in response to factors specific for a particular operon, in this case the availability of lactose. Global regulation, discussed later, is regulation in response to more general conditions, such as the overall carbon and energy supply of the cell.

The lactose or lac operon consists of three structural genes, lacZ, lacY, and lacA, together with an upstream regulatory region (Fig. 9.11). The lacZ structural gene encodes b-galactosidase, the enzyme that degrades lactose. The lacY gene encodes lactose permease (a transport protein) and lacA encodes lactose acetylase, whose role is not known (it is not needed for growth on lactose by E. coli). The lac operon is regulated by the LacI repressor protein, which is encoded by the lacI gene. This lies upstream of lacZYA and is transcribed in the opposite direction.

beta-galactosidase or ß-galactosidase (LacZ) Enzyme that splits lactose and related molecules to release galactose global regulation Regulation of a large group of genes in response to the same stimulus LacI The lactose repressor protein lactose permease (LacY) The transport protein for lactose

FIGURE 9.12 Specific Regulation of the lac Operon

When LacI binds to the operator site, no transcription takes place. The presence of the inducer can remove LacI from the operator, allowing RNA polymerase to bind and transcribe the operon. The inducer may be a//o-lactose, derived from lactose or an artificial compound, such as IPTG.

Inactive DNA

lac I repressor I

Promoter lac Z lac Y lac A +1

Operator site \Inducer

FIGURE 9.12 Specific Regulation of the lac Operon

When LacI binds to the operator site, no transcription takes place. The presence of the inducer can remove LacI from the operator, allowing RNA polymerase to bind and transcribe the operon. The inducer may be a//o-lactose, derived from lactose or an artificial compound, such as IPTG.

IPTG is an artificial inducer of the lactose operon.

The upstream region contains a recognition sequence for the repressor protein, known as the operator (lacO in Fig. 9.11). If no inducer is present, LacI protein binds to the operator. This blocks the binding of RNA polymerase to the promoter. When lactose is present, it induces the lac operon. The actual inducer is not lactose itself, but allo-lactose, an isomer of lactose that is made from lactose by b-galactosidase. The LacI repressor binds inducer and changes shape to its inactive form, which cannot bind DNA. RNA polymerase is now able to bind to the promoter and transcribe the lac operon (Fig. 9.12). LacI protein exists as a tetramer that can, in fact, bind two DNA recognition sites if these are available in the promoter region. This will result in looping of the DNA—see below. [Unlike many allosteric proteins, the LacI protein does not alternate between monomer and tetramer forms, but exists as a tetramer of unusual stability under all physiological conditions.]

In the laboratory, the lac operon is often induced by the compound IPTG (iso-propyl-thiogalactoside) (Fig. 9.13). This artificial compound is known as a gratuitous inducer because it is not metabolized by the products of the genes it induces. In this particular case, although IPTG induces the lacZYA genes, it is not broken down by b-galactosidase, the enzyme that degrades lactose. Consequently, IPTG continues to induce the lac operon long-term, whereas, natural inducers only induce for a short period of time before they are broken down.

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