Lactose Present

Tryptophan blocks RNA polymerase from binding and transcribing the structural genes, preventing synthesis of tryptophan pathway enzymes.

13.18 The trp Operon: A Repressible System Because tryptophan activates an otherwise inactive repressor, it is called a corepressor.

13.18 The trp Operon: A Repressible System Because tryptophan activates an otherwise inactive repressor, it is called a corepressor.

repressor) to render it incapable of binding to the operator, thus allowing transcription.

► In repressible systems, the product of a metabolic pathway (the corepressor) interacts with a regulatory protein to make it capable of binding to the operator, thus blocking transcription.

In general, inducible systems control catabolic pathways (which are turned on only when the substrate is available), whereas repressible systems control biosynthetic pathways (which are turned off until the product becomes unavailable). In both kinds of systems, the regulatory molecule functions by binding to the operator. Next, we will consider an example of control by binding to the promoter.

Protein synthesis can be controlled by increasing promoter efficiency

Suppose an E. coli cell lacks a supply of glucose, its preferred energy source, but instead has access to another sugar (such as lactose) that it can break down to obtain energy. Operons encoding enzymes that catabolize such alternative energy sources, such as the lac operon, have a mechanism for increasing the transcription of these enzymes by increasing the efficiency of the promoter. In these operons, the promoter binds RNA polymerase in a series of steps (Figure 13.19). First, a protein called CRP (short for cAMP receptor protein) binds the low-molecular-weight compound adenosine 3',5'-cyclic monophosphate, better known as cyclic AMP, or cAMP. Next, the CRP-cAMP complex binds to DNA just upstream (5') of the promoter. This binding results in more efficient binding of RNA polymerase to the promoter, and thus an elevated level of transcription of the structural genes.

When glucose becomes abundant in the medium, the bacterium does not need to break down alternative food molecules, so synthesis of the enzymes that catabolize these molecules diminishes or ceases. The presence of glucose decreases the synthesis of the enzymes by lowering the cellular concentration of cAMP. The lower cAMP concentration leads to less CRP binding to the promoter, less efficient binding of RNA polymerase, and reduced transcription of the structural genes. This mechanism is called catabolite repression.

As you will see in later chapters of this book, cAMP is a widely used signaling molecule in eukaryotes, as well as in prokaryotes. The use of this nucleotide in such widely diverse situations as a bacterium sensing glucose levels and a human sensing hunger demonstrates the prevalence of common themes in biochemistry and natural selection.

13.19 Transcription Is Enhanced by the Binding of the CRP-cAMP Complex to the Promoter The structural genes of this operon encode enzymes that break down a food source other than glucose.

The inducible lac and repressible trp systems—the two op-erator-repressor systems—are examples of negative control of transcription because the regulatory molecule (the repres-sor) in each case prevents transcription. The promoter-catabolite repression system is an example of positive control of transcription because the regulatory molecule (the CRP-cAMP complex) enhances transcription. The relationships between these positive and negative control systems are summarized in Table 13.2.

The control of gene expression by regulatory proteins is not unique to prokaryotes. As we will see in the next chapter, it also occurs in eukaryotes and even, as we are about to see, in viruses.

Control of Transcription in Viruses

The mechanisms used by used by viruses within a host cell for the regulation of gene expression are similar to those used by prokaryotes. Even a "simple" biological agent such as a virus is faced with complicated molecular decisions when its genome enters a cell. For example, the viral genome must di-

Low glucose, lactose present O cAMP

Low glucose, lactose present O cAMP

I When glucose levels are low, a receptor protein (CRP) and cAMP complex binds to the promoter, activating it.

Crp Camp Complex
High glucose, lactose present

ll When glucose levels are '

high, RNA polymerase cannot bind efficiently and transcription is reduced. \_J

2| Structural genes are transcribed at low levels. This is adaptive when the cell has enough glucose and does not need to break down lactose.

I When glucose levels are low, a receptor protein (CRP) and cAMP complex binds to the promoter, activating it.

| RNA polymerase then binds more efficiently to the promoter...

| RNA polymerase then binds more efficiently to the promoter...

ll When glucose levels are '

high, RNA polymerase cannot bind efficiently and transcription is reduced. \_J

13.1 Positive and Negative Controls in the lac Operona

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • karla rall
    When lactose is present and glucose is low camp binds to crp?
    8 years ago
  • sanna
    When glucose levels are high and lactose is present?
    8 years ago

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