Specific versus Global Control

Specific regulation refers to control by a signal specific for a small group of genes. Thus, lactose induces the lac operon, maltose induces the mal operon, etc. Global regulators control large numbers of genes in response to a more general signal or stimulus. Most global regulator A regulator that controls a large group of genes, generally in response to some stimulus or developmental stage specific regulation Regulation that applies to a single gene or operon or to a very small number of related genes

Crp Protein Is an Example of a Global Control Protein 255

FIGURE 9.22 Cyclic AMP and the Crp Global Regulator

Individual Crp units bind cyclic AMP to form a dimer that has the ability to bind DNA.

Cyclic AMP

Cyclic AMP

FIGURE 9.22 Cyclic AMP and the Crp Global Regulator

Individual Crp units bind cyclic AMP to form a dimer that has the ability to bind DNA.

DNA binding site is revealed

genes respond to both specific and global signals. Thus, in addition to specific control by the lac repressor, the lac operon is regulated by the global activator protein, Crp (Cyclic AMP Receptor Protein). The maltose genes are also regulated by Crp, which regulates the selection among different sugars.

Many bacteria can grow on a wide range of sugars, such as fructose (fruit sugar), lactose (milk sugar) and maltose (from starch breakdown), as well as glucose. When a preferred sugar, such as glucose, is present, less favored sugars, such as fructose, lactose or maltose, are not used. Only when glucose runs out will the other sugars be consumed. In molecular terms, this means the genes for using these other sugars are switched off when glucose is available.

A regulon is a group of several genes or operons that are turned on or off in response to the same signal by the same regulatory protein. The members of a regulon have separate promoters and are widely separated on the chromosome. Two examples of regulons in E. coli are the genes for using maltose and the genes for the synthesis of arginine. The arginine regulon consists of a dozen genes for biosynthesis and transport scattered over nine locations on the chromosome. They are controlled by a repres-sor, ArgR, which binds arginine as co-repressor, and is unlinked to any of the genes it controls.

The global regulator Crp binds the signal molecule, cyclic AMP.

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