A

Arabinose

Activator

Repressor

Activator

B. araBAD genes ON

Site 3

C. araBAD genes OFF

C. araBAD genes OFF

FIGURE 9.16 AraC Repressor and Activator

A) AraC dimers are either activators or repressors depending on whether arabinose is bound or not. B) When AraC binds arabinose, the dimer changes configuration and binds to DNA at sites 1 and 2. Here it acts as an activator, allowing the RNA polymerase to bind. C) When the repressor form of AraC binds DNA, it occupies sites 2 and 3, forming a loop in the DNA and causing gene inactivation.

Repressor subunits. Again, maltose itself is not the true inducer. The MalT protein actually binds maltotriose, a trisaccharide consisting of three glucose residues.

Some repressors are only active when they bind a small signal molecule called a co-repressor. This situation is often found when regulating biosynthetic pathways. If an amino acid, such as tryptophan, is present in the culture medium, then the cell does not need to make it. On the other hand, if the amino acid is not present in sufficient amounts, the pathway for synthesis needs to be turned on. In general, the cell should turn biosynthetic pathways off when their products are present in the medium or have been synthesized in sufficient amounts. Thus biosynthetic pathways respond to the corresponding nutrient. An example is the ArgR repressor of E. coli, which binds the amino acid arginine as its co-repressor (Fig. 9.17).

The signal molecule itself is not always small. Sometimes repressors or activators bind other proteins, rather than small metabolites. For example, the Mlc repressor regulates glucose transport and a variety of other genes involved in the uptake and metabolism of monosaccharides. The Mlc protein does not bind glucose, yet responds to its presence indirectly. When glucose enters the cell, it is converted to glucose-6-phosphate by the PtsG transporter. When glucose is absent, phosphate groups accumulate on PtsG protein. Conversely, when glucose is present, PtsG rapidly transfers the phosphate to glucose and most PtsG protein is therefore non-phosphorylated. This form of PtsG binds to Mlc and prevents it from binding to DNA. The inactive Mlc protein is thus found attached to the cell membrane where PtsG is located (Fig. 9.18).

co-repressor In prokaryotes—a small signal molecule needed for some repressor proteins to bind to DNA;in eukaryotes—an accessory protein, often a histone deacetylase, involved in gene repression

FIGURE 9.17 ArgR Repressor Uses Arginine as a Co-repressor

In the absence of high levels of arginine the ArgR repressor cannot bind to DNA. Therefore RNA polymerase is active in transcribing the genes for the synthesis of arginine. When sufficient arginine is present, the arginine acts as a co-repressor by binding to ArgR. The complex then binds to the double operator sites and represses the genes for the synthesis of arginine.

FIGURE 9.17 ArgR Repressor Uses Arginine as a Co-repressor

In the absence of high levels of arginine the ArgR repressor cannot bind to DNA. Therefore RNA polymerase is active in transcribing the genes for the synthesis of arginine. When sufficient arginine is present, the arginine acts as a co-repressor by binding to ArgR. The complex then binds to the double operator sites and represses the genes for the synthesis of arginine.

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