Module 4 Exam

Constitutive Genes

* A gene that is expressed continually without regulation. * Bacteria and eukaryotes use regulatory genes to control the expression of many of their structural genes. However, a few structural genes, particularly those that encode essential cellular functions (often called housekeeping genes) are expressed continually and are therefore said to be constitutive. Constitutive genes are not regulated.

2nd Level of Regulation

* A second point at which a gene can be regulated is at the level of transcription. For the sake of cellular economy, limiting the production of a protein early in the process makes sense, and transcription is an important point of gene regulation in both bacterial and eukaryotic cells.

Gene Regulation Concept Check

* In bacteria, gene regulation maintains internal flexibility, turning genes on and off in response to environmental changes. In multicellular eukaryotic organisms, gene regulation also brings about cell differentiation.

Positive Control

* In positive transcriptional control, the regulatory protein involved is an activator: it binds to DNA (usually at a site other than the operator) and stimulates transcription. For example, in the case of the lac operon, the catabolite activator protein (CAP) binds to the promoter and increases the efficiency with which RNA polymerase can bind the promoter and transcribe the structural genes (as we'll see later in this section).

Lactose Metabolism

* Lactose is a major carbohydrate found in milk; it can be metabolized by E. coli bacteria that reside in the mammalian gut. Lactose does not easily diffuse across the E. coli cell membrane and must be actively transported into the cell by the protein lactose permease (Figure 16.6). To use lactose as an energy source, E. coli must then break it into glucose and galactose, a reaction catalyzed by the enzyme β-galactosidase. This enzyme can also convert lactose into allolactose, a compound that plays an important role in regulating lactose metabolism. A third enzyme, thiogalactoside transacetylase, is also produced by the lac operon, but its function in lactose metabolism is not yet clear. One possible function is detoxification, preventing the accumulation of thiogalactosides that are transported into the cell along with lactose by permease.

Concept Check

* There are two basic types of transcriptional control: negative and positive. In negative control, when a regulator protein (repressor) binds to DNA, transcription is inhibited; in positive control, when a regulator protein (activator) binds to DNA, transcription is stimulated. Some operons are inducible: their transcription is normally off and must be turned on. Other operons are repressible: their transcription is normally on and must be turned off.

Domain

* These regulatory proteins generally have discrete functional parts—called domains, typically consisting of 60 to 90 amino acids—that are responsible for binding to DNA.

Attentuation Summary

* To summarize, the 5′ UTR of the trp operon can fold into one of two secondary structures. When the tryptophan level is high, the 3+4 structure forms, transcription is terminated within the 5′ UTR, and no additional tryptophan is synthesized. When the tryptophan level is low, the 2+3 structure forms, transcription continues through the structural genes, and tryptophan is synthesized. The critical question, then, is this: Why does the 3+4 structure arise when the level of tryptophan in the cell is high, whereas the 2+3 structure arises when the level is low?

Attenuation

* Type of gene regulation in some bacterial operons in which transcription is initiated but terminates prematurely before the transcription of the structural genes.

16.10

16.10 The partial diploid lacI s IacZ+/lacI+ lacZ+. fails to produce β-galactosidase in the presence and absence of lactose because the lacIs gene encodes a superrepressor.

16.9

16.9 The partial diploid lacI+ lacZ−/lacI− lacZ+ produces β-galactosidase only in the presence of lactose because the lacI gene is trans dominant.

Coreprosser

Substance that inhibits transcription in a repressible system of gene regulation; usually a small molecule that binds to a repressor protein and alters it so that the repressor is able to bind to DNA and inhibit transcription.

Inducer

Substance that stimulates transcription in an inducible system of gene regulation; usually a small molecule that binds to a repressor protein and alters that repressor so that it can no longer bind to DNA and inhibit transcription.

Negative and Positive Control: Inducible and Repressible Operons

There are two types of transcriptional control: negative control, in which a regulatory protein is a repressor, binding to DNA and inhibiting transcription; and positive control, in which a regulatory protein is an activator, stimulating transcription. Operons can also be either inducible or repressible. Inducible operons are those in which transcription is normally off (not taking place); something must happen to induce transcription, or turn it on. Repressible operons are those in which transcription is normally on (taking place); something must happen to repress transcription, or turn it off. In the next few sections, we will consider several varieties of these basic control mechanisms.

4th Level of Regulation

* A fourth point for the control of gene expression is the regulation of mRNA stability. The amount of protein produced depends not only on the amount of mRNA synthesized, but also on the rate at which the mRNA is degraded.

What is a Constitutive Gene?

* A gene that is expressed continually without regulation.

How do amino acids in DNA-binding proteins interact with DNA?

* By forming hydrogen bonds with DNA bases

1st level of Regulation

* First, genes can be regulated through the alteration of DNA or chromatin structure; this type of gene regulation takes place primarily in eukaryotes. Modifications to DNA or its packaging can help to determine which sequences are available for transcription or the rate at which sequences are transcribed. DNA methylation and changes in chromatin structure are two processes that play a pivotal role in gene regulation.

Why is transcription a particularly important level of gene regulation in both bacteria and eukaryotes?

* For the sake of cellular economy, limiting the production of a protein early in the process makes sense

Regulator Gene Mutations

* Jacob and Monod also isolated mutations that affected the regulation of protein production. Mutations in the lacI gene affect the production of both β-galactosidase and permease because the genes for both proteins are in the same operon and are regulated coordinately by the lac repressor protein. * Some of these mutations were constitutive, causing the lac proteins to be produced all the time, whether lactose was present or not. Such mutations in the regulator gene were designated lacI−. The construction of partial diploids demonstrated that a lacI+ gene is dominant over a lacI− gene; a single copy of lacI+ (genotype lacI+/ lacI−) was sufficient to bring about normal regulation of protein production. Furthermore, lacI+ restored normal control to an operon even if the operon was located on a different DNA molecule, showing that lacI+ can be trans acting. A partial diploid with genotype lacI+lacZ−/ lacI−lacZ+ functioned normally, synthesizing β-galactosidase only when lactose was present (Figure 16.9). In this strain, the lacI+ gene on the bacterial chromosome was functional, but the lacZ− gene was defective; on the plasmid, the lacI− gene was defective, but the lacZ+ gene was functional. The fact that a lacI+ gene could regulate a lacZ+ gene located on a different DNA molecule indicated to Jacob and Monod that the lacI+ gene product was able to operate on either the plasmid or the chromosome. Some lacI mutations isolated by Jacob and Monod prevented transcription from taking place even in the presence of lactose. These mutations were referred to as superrepressors (lacIs) because they produced defective repressor proteins that could not be inactivated by an inducer. A lacIs mutation produced a repressor with an altered inducer-binding site; consequently, the inducer was unable to bind to the repressor, and the repressor was always able to attach to the operator and prevent transcription of the lac genes. * Superrepressor mutations were dominant over lacI+; partial diploids with genotype lacIslacZ+lacY+/ lacI+lacZ+lacY+ were unable to synthesize either β-galactosidase or permease, whether or not lactose was present (Figure 16.10).

Inducible Operon

* Operon in which transcription is normally turned off, so that something must take place for transcription to be induced, or turned on.

Repressible Operon

* Operon in which transcription is normally turned on, so that something must take place for transcription to be repressed, or turned off.

Antiterminator (2+3 secondary structure)

* Protein or DNA sequence that inhibits the termination of transcription.

epigenetics

* phenotypes and processes that are transmitted to other cells and sometimes future generations, but are not the result of differences in the DNA base sequence

5th Point Level of Regulation

* A fifth point of gene regulation is at the level of translation, a complex process requiring a large number of enzymes, protein factors, and RNA molecules (see Chapter 15). All of these factors, as well as the availability of amino acids, affect the rate at which proteins are produced and therefore provide points at which gene expression can be controlled. Translation can also be affected by sequences in mRNA, such as those in the 5′ and 3′ untranslated regions. * Finally, many proteins are modified after translation (see Chapter 15), and these modifications affect whether the proteins become active; therefore, genes can be regulated through processes that affect posttranslational modification. Gene expression can be affected by regulatory activities at any or all of these points.

The Regulation of Gene Expression Is Critical for All Organisms

* A major theme of molecular genetics is the central dogma, which states that genetic information flows from DNA to RNA to proteins * Consider E. coli, a bacterium that resides in your large intestine. Your eating habits completely determine the nutrients available to this bacterium: it can neither seek out nourishment when nutrients are scarce nor move away when confronted with an unfavorable environment. The bacterium makes up for its inability to alter the external environment by being internally flexible. For example, if glucose is present, E. coli uses it to generate ATP; if there's no glucose, it uses lactose, arabinose, maltose, xylose, or any of a number of other sugars. When amino acids are available, E. coli uses them to synthesize proteins; if a particular amino acid is absent, E. coli produces the enzymes needed to synthesize that amino acid. Thus, E. coli responds to environmental changes by rapidly altering its biochemistry. This biochemical flexibility, however, has a high price. Constantly producing all the enzymes necessary for every environmental condition would be energetically expensive. So how does E. coli maintain biochemical flexibility while optimizing energy efficiency? The answer is through gene regulation. * Bacteria carry the genetic information for synthesizing many proteins, but only a subset of that genetic information is expressed at any time. When the environment changes, new genes are expressed, and proteins appropriate for the new environment are synthesized. For example, if a carbon source appears in the environment, genes encoding enzymes that take up and metabolize that carbon source are quickly transcribed and translated. When that carbon source disappears, the genes that encode those enzymes are shut off. Multicellular eukaryotic organisms face a further challenge. Individual cells in a multicellular organism are specialized for particular tasks. The proteins produced by a nerve cell, for example, are quite different from those produced by a white blood cell. But although they differ in shape and function, a nerve cell and a white blood cell still carry the same genetic instructions. * A multicellular organism's challenge is to bring about the differentiation of cells that have a common set of genetic instructions (the process of development). This challenge is met through gene regulation: all of an organism's cells carry the same genetic information, but only a subset of genes are expressed in each cell type; genes needed for other cell types are not expressed. Gene regulation is therefore the key to both unicellular flexibility and multicellular specialization, and it is critical to the success of all living organisms.

Operons Control Transcription in Bacterial Cells

* A significant difference between bacterial and eukaryotic gene control lies in the organization of functionally related genes. As we saw in the introduction to this chapter, many bacterial genes that have related functions are clustered together and under the control of a single promoter. These genes are often transcribed together into a single mRNA molecule. A group of bacterial structural genes that are transcribed together, along with their promoter and additional sequences that control their transcription, is called an operon. The operon regulates the expression of the structural genes by controlling transcription, which, in bacteria, is usually the most important level of gene regulation.

3rd Level of Regulation

* A third potential point of gene regulation is mRNA processing. Eukaryotic mRNA is extensively modified before it is translated: a 5′ cap is added, the 3′ end is cleaved and polyadenylated, and introns are removed (see Chapter 14). These modifications determine the movement of the mRNA into the cytoplasm, whether the mRNA can be translated, the rate of translation, and the amino acid sequence of the protein produced, as well as mRNA stability. There is growing evidence that a number of regulatory mechanisms in eukaryotic cells operate at the level of mRNA processing. In prokaryotes, however, in which these types of mRNA processing are largely absent, control of gene expression occurs primarily at other points.

A RIBOSWITCH CONTROLS THE SYNTHESIS OF VITAMIN B12

* An example of a riboswitch is seen in the bacterial genes that encode enzymes that play roles in the synthesis of vitamin B12. The genes for these enzymes are transcribed into an mRNA molecule that contains a riboswitch. When the activated form of vitamin B12—called coenzyme B12—is present, it binds to the riboswitch, and the mRNA folds into a secondary structure that obstructs the ribosome-binding site. Consequently, no translation of the mRNA takes place. In the absence of coenzyme B12, the mRNA assumes a different secondary structure. This secondary structure does not obstruct the ribosome-binding site, so translation is initiated, the enzymes are synthesized, and vitamin B12 is produced. For some riboswitches, the regulatory molecule acts as a repressor (as just described) by inhibiting transcription or translation; for others, the regulatory molecule acts as an inducer by causing the formation of a secondary structure that allows transcription or translation to take place.

DNASE I Hypersensitivity

* As genes become transcriptionally active, regions around the genes become highly sensitive to the action of DNase I (see pp. 317-318 in Chapter 11). These regions, called DNase I hypersensitive sites, frequently develop about 1000 nucleotides upstream of the transcription start site, suggesting that the chromatin in these regions adopts a more open configuration during transcription. This relaxation of the chromatin structure allows regulatory proteins access to binding sites on the DNA. Indeed, many DNase I hypersensitive sites correspond to known binding sites for regulatory proteins. At least three different processes affect gene regulation by altering chromatin structure: (1) chromatin remodeling; (2) the modification of histone proteins; and (3) DNA methylation. Each of these mechanisms will be discussed in the sections that follow

In the presence of allolactose, the lac repressor

* Cannot bind to operator

Operator

* DNA sequence in an operon of a bacterial cell to which a regulator protein binds; this binding affects the rate of transcription of the structural genes.

Regulatory Elements

* DNA sequence that affects the transcription of other DNA sequences to which it is physically linked. * We will also encounter DNA sequences that are not transcribed at all, but still play a role in regulating genes and other DNA sequences. These regulatory elements affect the expression of DNA sequences to which they are physically linked. Regulatory elements are common in both bacterial and eukaryotic cells, and much of gene regulation in both types of organisms takes place through the action of proteins produced by regulatory genes that recognize and bind to regulatory elements. * The regulation of gene expression can occur through processes that stimulate gene expression, termed positive control, or through processes that inhibit gene expression, termed negative control. Bacteria and eukaryotes use both positive and negative control mechanisms to regulate their genes.

Regulatory Genes

* DNA sequence that encodes a protein or RNA molecule that interacts with DNA sequences and affects their transcription or translation or both. * either RNA or proteins—interact with other DNA sequences and affect the transcription or translation of those sequences. In many cases, the products of regulatory genes are DNA-binding proteins or RNA molecules that affect gene expression.

Structural Genes

* DNA sequence that encodes a protein that functions in metabolism or biosynthesis or that has a structural role in the cell.

The lac OPERON of E Coli

* François Jacob and Jacques Monod first described the "operon model" for the genetic control of lactose metabolism in E. coli in 1961. Their work, and subsequent research on the genetics of lactose metabolism, established the operon as the basic unit of transcriptional control in bacteria. Despite the fact that, at the time, no methods were available for determining nucleotide sequences, Jacob and Monod deduced the structure of the operon genetically by analyzing the interactions of mutations that interfered with the normal regulation of lactose metabolism. We examine the effects of some of these mutations after seeing how the lac operon regulates lactose metabolism.

Concept Check

* Functionally related genes in bacterial cells are frequently clustered together in a single transcription unit termed an operon. A typical operon includes several structural genes, a promoter for those structural genes, and an operator to which the product of a regulator gene binds.

Regulator Gene

* Gene associated with an operon in bacterial cells that encodes a protein or RNA molecule that functions in controlling the transcription of one or more structural genes.

Concept Check

* Gene expression can be controlled at any of a number of levels along the molecular pathway from DNA to protein, including DNA or chromatin structure, transcription, mRNA processing, mRNA stability, translation, and posttranslational modification.

Negative Control

* Gene regulation in which the binding of a regulatory protein to DNA inhibits transcription (the regulatory protein is a repressor).

Positive Control

* Gene regulation in which the binding of a regulatory protein to DNA stimulates transcription (the regulatory protein is an activator).

CC

* In spite of its name, catabolite repression is a type of positive control. The catabolite activator protein (CAP), complexed with cAMP, binds to a site at or near the lac promoter and stimulates the binding of RNA polymerase. Cellular levels of cAMP are controlled by glucose; a low glucose level increases the abundance of cAMP and enhances the transcription of the lac structural genes.

The regulator protein that acts on a negative repressible operon is synthesized as

* Inactive Repressor

Structural Gene Mutations (LacZ+LacY-/LacZ-LacY+)

* Jacob and Monod first discovered some mutant strains that had lost the ability to synthesize either β-galactosidase or permease (they did not study in detail the effects of mutations on the transacetylase enzyme, so transacetylase will not be considered here). The mutations in those mutant strains mapped to the lacZ or lacY structural genes and altered the amino acid sequences of the proteins encoded by the genes. These mutations clearly affected the structure of the proteins, but not the regulation of their synthesis. * Through the use of partial diploids, Jacob and Monod were able to establish that mutations of the lacZ and lacY genes were independent and usually affected only the product of the gene in which the mutation occurred. Partial diploids with lacZ+lacY− on the bacterial chromosome and lacZ−lacY+ on the plasmid functioned normally, producing β-galactosidase and permease in the presence of lactose. (The genotype of a partial diploid is written by separating the genes on each DNA molecule with a slash: lacZ+lacY−/ lacZ−lacY+.) In these partial diploids, a single functional β-galactosidase gene (lacZ+) was sufficient to produce β-galactosidase; whether the functional β- galactosidase gene was coupled to a functional (lacY+) or a defective (lacY−) permease gene made no difference. The same was true of the lacY+ gene.

lac Mutations

* Jacob and Monod worked out the structure and function of the lac operon by analyzing mutations that affected lactose metabolism. To help define the roles of the different components of the operon, they used partial-diploid strains of E. coli. The cells of these strains possessed two different DNA molecules: the full bacterial chromosome and an extra piece of DNA. Jacob and Monod created these strains by allowing conjugation to take place between two bacteria. In conjugation, a small circular piece of DNA (the F plasmid) is transferred from one bacterium to another (see Chapter 9). The F plasmid used by Jacob and Monod contained the lac operon, so the recipient bacterium became partly diploid, possessing two copies of the lac operon. By using different combinations of mutations on the bacterial and plasmid DNA, Jacob and Monod determined that some parts of the lac operon are cis acting (able to control the expression of genes only on the same piece of DNA), whereas other parts are trans acting (able to control the expression of genes on other DNA molecules).

DNA Binding Proteins

* Much of gene regulation in bacteria and eukaryotes is accomplished by proteins that bind to DNA sequences and affect their expression. These regulatory proteins generally have discrete functional parts—called domains, typically consisting of 60 to 90 amino acids—that are responsible for binding to DNA. Within a domain, only a few amino acids actually make contact with the DNA. These amino acids (most commonly asparagine, glutamine, glycine, lysine, and arginine) often form hydrogen bonds with the bases or interact with the sugar-phosphate backbone of the DNA. Many DNA-binding proteins have additional domains that can bind other molecules, such as other regulatory proteins. By physically attaching to DNA, these proteins can affect the expression of a gene. Most DNA-binding proteins bind dynamically, which means that they transiently bind and unbind DNA and other regulatory proteins. Thus, although they may spend most of their time bound to DNA, they are never permanently attached. This dynamic nature means that other molecules can compete with DNA-binding proteins for regulatory sites on the DNA. * DNA-binding proteins can be grouped into several distinct types on the basis of characteristic structures, called motifs, found within the binding domain. Motifs are simple structures, such as alpha helices, that can fit into the major groove of the DNA double helix. For example, the helix-turn-helix motif (Figure 16.2a), consisting of two alpha helices connected by a turn, is common in bacterial regulatory proteins. The zinc-finger motif (Figure 16.2b), shared by many eukaryotic regulatory proteins, consists of a loop of amino acids containing a zinc ion. The leucine zipper (Figure 16.2c) is another motif found in a variety of eukaryotic DNA-binding proteins. These common DNA-binding motifs and others are summarized in Table 16.1.

Regulator Protein

* Protein produced by a regulator gene that binds to another DNA sequence and controls the transcription of one or more structural genes.

Concept Check

* Regulatory proteins that bind DNA have common motifs that interact with the double-helical structure of DNA.

Attentuator (1+2 and 3+4 secondary structure)

* Secondary structure that forms in the 5′ untranslated region of some operons and causes the premature termination of transcription.

Operon

* Set of structural genes in a bacterial cell, along with their common promoter and other sequences (such as an operator) that control their transcription. * The operon regulates the expression of the structural genes by controlling transcription, which, in bacteria, is usually the most important level of gene regulation.

Other Sequences Control the Expression of Some Bacterial Genes

* Several types of sequences outside of operons affect the expression of some genes in bacteria. These sequences include bacterial enhancers and several types of RNA regulators.

NEGATIVE REPRESSIBLE OPERONS

* Some operons with negative control are repressible, meaning that transcription normally takes place and must be turned off, or repressed. The regulator protein that acts on this type of operon is also a repressor, but it is synthesized in an inactive form that cannot by itself bind to the operator. Because no repressor is bound to the operator, RNA polymerase readily binds to the promoter, and transcription of the structural genes takes place (Figure 16.5a). * To turn transcription off, something must happen to make the repressor active. When a small molecule called a corepressor is present, it binds to the repressor and makes it capable of binding to the operator. In the example illustrated (see Figure 16.5a), the product (U) of the metabolic reaction controlled by the operon is the corepressor. As long as the level of product U is high, it is available to bind to the repressor and activate it, preventing transcription (Figure 16.5b). With the operon repressed, enzymes G, H, and I are not synthesized, and no more U is produced from precursor T. However, when all of product U is used up, the repressor is no longer activated by product U and cannot bind to the operator. The inactivation of the repressor allows the transcription of the structural genes and the synthesis of enzymes G, H, and I, resulting in the conversion of precursor T into product U. Like inducible operons, repressible operons are economical: the proteins they encode are synthesized only as needed. * Repressible operons usually control proteins that carry out the biosynthesis of molecules needed in the cell, such as amino acids. For these types of operons, repressible control makes sense because the product produced by the proteins is always needed by the cell. Thus, these operons are normally on and are turned off only when there are adequate amounts of the product already present. * Note that both the inducible and the repressible operon systems that we have just considered are forms of negative control, in which the regulator protein is a repressor.

Concept Check

* Structural genes encode proteins, while regulatory elements are DNA sequences that are not transcribed, but affect the expression of genes. Positive control mechanisms stimulate gene expression, whereas negative control mechanisms inhibit gene expression.

What is the difference between a structural gene and a regulator gene?

* Structural genes encode proteins; regulator genes control the transcription of structural genes.

CAP Repression

* System of gene control in some bacterial operons in which glucose is used preferentially and the metabolism of other sugars is repressed in the presence of glucose.

Concept Check

* The lac operon of E. coli controls the transcription of three genes needed in lactose metabolism: the lacZ gene, which encodes β-galactosidase; the lacY gene, which encodes lactose permease; and the lacA gene, which encodes thiogalactoside transacetylase. The lac operon is a negative inducible operon: a regulator gene produces a repressor that binds to the operator and prevents the transcription of the structural genes. The presence of allolactose inactivates the repressor and allows the transcription of the lac operon.

Operon Structure

* The organization of a typical operon is illustrated in Figure 16.3. At one end of the operon is a set of structural genes, shown in Figure 16.3 as gene a, gene b, and gene c. These structural genes are transcribed into a single mRNA, which is translated to produce enzymes A, B, and C. These enzymes carry out a series of biochemical reactions that convert precursor molecule X into product Y. The transcription of structural genes a, b, and c is under the control of a single promoter, which lies upstream of the first structural gene. RNA polymerase binds to the promoter and then moves downstream, transcribing the structural genes. * A regulator gene helps to control the expression of the structural genes of the operon by increasing or decreasing their transcription. Although it affects operon function, the regulator gene is not considered part of the operon. The regulator gene has its own promoter and is transcribed into a short mRNA, which is translated into a small protein. This regulator protein can bind to a region of the operon called the operator and affect whether transcription can take place. The operator usually overlaps the 3′ end of the promoter and sometimes the 5′ end of the first structural gene (see Figure 16.3).

NEGATIVE INDUCIBLE OPERON

* The regulator gene for a negative inducible operon encodes an active repressor protein that readily binds to the operator (Figure 16.4a). Because the operator site overlaps the promoter site, the binding of this protein to the operator physically blocks the binding of RNA polymerase to the promoter and prevents transcription. For transcription to take place, something must happen to prevent the binding of the repressor to the operator. This type of system is said to be inducible because transcription is normally off (inhibited) and must be turned on (induced). * Transcription of a negative inducible operon is turned on when a small molecule called an inducer is present and binds to the repressor (Figure 16.4b). Regulatory proteins frequently have two binding sites: one that binds to DNA and another that binds to a small molecule such as an inducer. The binding of the inducer (precursor V in Figure 16.4b) alters the shape of the repressor, preventing it from binding to DNA. Proteins such as this repressor, which change shape upon binding to another molecule, are called allosteric proteins. * When the inducer is absent, the repressor binds to the operator, the structural genes are not transcribed, and enzymes D, E, and F (which metabolize precursor V) are not synthesized (see Figure 16.4a). This mechanism is an adaptive one: because no precursor V is present, synthesis of the enzymes would be wasteful because they would have no substrate to metabolize. As soon as precursor V becomes available, some of it binds to the repressor, rendering the repressor inactive and unable to bind to the operator. RNA polymerase can now bind to the promoter and transcribe the structural genes. The resulting mRNA is then translated into enzymes D, E, and F, which convert substrate V into product W (see Figure 16.4b). So, an operon with negative inducible control regulates the synthesis of the enzymes economically: the enzymes are synthesized only when their substrate (V) is available. * Inducible operons usually control proteins that carry out degradative processes—proteins that break down molecules. For these types of proteins, inducible control makes sense because the proteins are not needed unless the substrate (which is broken down by the proteins) is present.

Coordinate Induction

* The simultaneous synthesis of several enzymes stimulated by a single environmental factor. * Simultaneous synthesis of several proteins stimulated by a specific molecule, the inducer

CC

* The trp operon is a negative repressible operon that controls the biosynthesis of tryptophan. In a repressible operon, transcription is normally turned on and must be repressed: in the case of the trp operon, this is accomplished through the binding of tryptophan to the repressor, which renders the repressor active. The active repressor then binds to the operator and prevents RNA polymerase from transcribing the structural genes.