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as meningitis and typhoid

fever. Chloramphenicol readily enters human mitochondria, where it inhibits protein synthesis. Cells of

the bone marrow often fail to develop in patients treated with chloramphenicol, and use of this antibiotic

has been linked to fatal blood dyscrasias, including an aplastic anemia. Erythromycin. Erythromycin and the other macrolide antibiotics bind to the 50S ribosomal subunit

of bacteria near the binding site for chloramphenicol. They prevent the translocation step, the

movement of the peptidyl-tRNA from the A site to the P site on the ribosome. Because the side effects are

less severe and more readily reversible than those of many other antibiotics, the macrolides are often

used to treat infections in persons who are allergic to penicillin, an antibiotic that inhibits bacterial cellwall synthesis. However, bacterial resistance to erythromycin is increasing. Therefore, its close relative,

azithromycin, is often used. KEY CONCEP TS

Translation is the process of translating the sequence of nucleotides in mRNA to an amino acid

sequence of a protein.

Translation proceeds from the amino to the carboxyl terminus, reading the mRNA in the 5′-to-3direction′ .

Protein synthesis occurs on ribosomes.

The mRNA is read in codons, sets of three nucleotides that specify individual amino acids.

AUG, which specifies methionine, is the start codon for all protein synthesis. Specific stop codons (UAG, UGA, and UAA) signal when the translation of the mRNA is to end.

Amino acids are linked covalently to tRNA by the enzyme aminoacyl-tRNA synthetase, creating

charged tRNA.

Charged tRNAs base-pair with the codon via the anticodon region of the tRNA. Protein synthesis is divided into three stages: initiation, elongation, and termination.

Multiprotein factors are required for each stage of protein synthesis. Proteins fold as they are synthesized.

Specific amino acid side chains may be modified after translation by a process known as

posttranslational modification.

Mechanisms within eukaryotic cells specifically target newly synthesized proteins to different

compartments in the cell.

Table 15.7 summarizes the diseases discussed in this chapter.

REVIEW QUESTIONS—CHAPTER 151. Antibiotics can target differences in processing of the genetic code between bacteria and humans. In

the readout of the genetic code in prokaryotes, which one of the following processes acts before any

of the others?

A.tRNAi alignment with mRNA

B.Termination of transcription

C.Movement of the ribosome from one codon to the next

D.Recruitment of termination factors to the A site

E.Export of mRNA from the nucleus

2. Genetic mutations can be simulated in laboratory situations. tRNA charged with cysteine can be

chemically treated so that the amino acid changes its identity to alanine. If some of this charged

tRNA is added to a protein-synthesizing extract that contains all the normal components required for

translation, which of the following statements represents the most likely outcome after adding an

mRNA that has both Cys and Ala codons in the normal reading frame?

A. Cysteine would be added each time the alanine codon was translated.

B.Alanine would be added each time the cysteine codon was translated.

C.The protein would have a deficiency of cysteine residues.

D.The protein would have a deficiency of alanine residues.

E.The protein would be entirely normal.

3.Human and bacterial DNA differ in the organization of genetic information. A series of eukaryotic

gene sequences (coding sequences) is given below. Based on this portion of the sequence, which

gene could produce a protein that contains 300 amino acids and has a phenylalanine residue near its

N terminus? The phenylalanine codons (5′-to-3)′ are UUU and UUC. A. 5′-CCATGCCATTTGCATCA -3′

B. 5′-CCATGCCATTTGCATGA-3′ C. 5′-CCATGCCAATTTGCATC-3′ D. 5′-CCATCCCATTTGCATGA-3′ E. 5′-CCATCCCATTTGCATCA-3′

4.A drug is being designed to block eukaryotic translation. Which of the following would be an

appropriate target for the drug? Choose the one best answer.

5.A gene has undergone a mutation in which a certain codon has been altered, through a single

nucleotide change, into a Phe codon. Which one of the following amino acids could the original

codon code for? A. Pro

B. LeuC. Gly D. Asn

E. Arg F. Trp

6.A mutation in a gene has led to the generation of a nonsense codon in the corresponding protein. If

this is caused by a single nucleotide change, the original codon may have coded for which one of the

following amino acids? A. Gly

B. Pro C. Phe D. Asn E. Trp

7.Which type of mutation leads to sickle cell anemia? Choose the one best answer. A. Silent

B. Nonsense C. Insertion D. Deletion E. Missense

8.The creation of a stop codon in DNA often leads to a deleterious condition. If a mutation in DNA

caused a stop codon TAG to be created on the coding strand between the TATA box and the

transcription initiation site, what would be the most likely outcome? A. No effect

B. Loss of transcription C. Loss of translation D. A shorter protein

E. A mistake in splicing

9.A temperature-sensitive variant of Escherichia coli was discovered which had a complete loss of

protein synthetic ability after about five generations of growth at the nonpermissive temperature.

Analysis of the components required for protein synthesis indicated that mRNA synthesis

(transcription) still occurred at the nonpermissive temperature, and the mRNA produced had normal

half-lives. The transcription of rRNA and tRNA was also normal as was ribosome

structure. A

potential protein in which loss of activity at the nonpermissive temperature would lead to these

findings is which one of the following?

A.A spliceosome protein

B.A tRNA-modifying enzyme

C.The capping protein

D.Poly(A) polymerase

E.A nuclear mRNA export protein

10. A new patient, recently admitted to the hospital, has contracted diphtheria. A family history indicates

that the patient had never been vaccinated against this pathogen. Protein synthesis in the patient’s

cells is inhibited owing to which one of the following?A. An inhibition of RNA polymerase II activity

B.An inhibition of peptidyltransferase activity

C.An inhibition of the assembly of the translation-initiation complex

D.An inhibition of the translocation step of protein synthesis

E.An inhibition of the termination of protein synthesis.

ANSWERS TO REVIEW QUESTIONS

1.The answer is A. It is important to note that the question is asking about prokaryotic mechanisms.

In prokaryotes, there is no nucleus (thus, E cannot be correct), and translation begins before

transcription is terminated (coupled translation–transcription, thus B is incorrect). Therefore,

before the ribosome can move from one codon to the next (translocation), or the protein synthesis

machinery terminates (via termination factors), the initiating tRNA must bind and align with the

mRNA to initiate translation, indicating that answer A is the first step of the choices listed that

must occur.

2.The answer is C. Because the extract contains all normal components, the Cys-tRNA charged

with alanine will compete with Cys-tRNA charged with cysteine for binding to the cysteine

codons. Thus, the protein will have some alanines put in the place of cysteine, leading to a

deficiency of cysteine residues. Answer A is incorrect because the tRNA recognizes the cysteine

codon, not the alanine codon. Answer B is incorrect because of the competition mentioned

previously.

3.The answer is A. In order to answer this question, one first needs to find the start codon: AUG in

mRNA and ATG in DNA. Bases 3 to 5 in the sequence contain this element. Next, the sequence

needs to specify a phenylalanine residue near the amino terminus, which is UUU or UUC in

mRNA, or TTT or TTG in DNA, but these sequences need to be in frame with the initiating

methionine codon. The last element to look for is the absence of a premature stop codon because

this protein is 300 amino acids long. There is no stop sequence in the remaining bases of this

piece of DNA. For answer B, the last three bases of this sequence are TGA, which in mRNA

would be UGA, and this is an in-frame stop signal. This sequence would not give rise to a protein

that contained 300 amino acids. For answer C, the TTT in that sequence (bases 10 to 12) are not

in frame with the initiating methionine, indicating that there is not a phenylalanine near the amino

terminus in the protein encoded by this sequence. For answers D and E, there are no

ATG

sequences, indicating that the initiating methionine is absent and that this stretch of DNA cannot

code for the amino-terminal end of a protein.

4. The answer is F. N-Formyl-methionine is used for the initiation of prokaryotic protein synthesis

but not eukaryotic protein synthesis. In eukaryotes, the mRNA is processed within the nucleus and

then exported into the cytoplasm before ribosomes can bind to it, so there is no nuclear mRNA–

ribosome complex formed. Splicing occurs in the nucleus, but splicing errors will not alter

translation. Eukaryotic translation requires an initiator tRNA for methionine, so if that tRNA were

unavailable, there would be no eukaryotic translation occurring. The cap on eukaryotic mRNA is

necessary for appropriate IF binding to allow the charged initiator tRNA and ribosomes to bind to

the mRNA. In the absence of cap formation, eukaryotic translation would be inhibited.5. The answer is B. The codons for Phe are UUC and UUU. Leucine has six codons, and single

nucleotide changes in four of those codons would result in a Phe codon. This is not the case for

Pro, Gly, Asn, or Arg codons; in order to convert those codons to a Phe codon, two nucleotide

changes are required. Answering this question required consultation with the genetic code.

6.The answer is E. A nonsense codon is the conversion of a codon to a stop codon. The stop

codons are UGA, UAG, and UAA. The codon for Trp is UGG, which can be converted to UGA or

UAG with a single nucleotide change. None of the other amino acid codons, with a single

nucleotide change, can be converted into a stop codon.

7.The answer is E. A single mutation of one amino acid (valine for glutamate) causes sickle cell

anemia. This change specifies a different amino acid (missense). A silent mutation specifies the

same amino acid and has no consequence. A nonsense mutation is a change to a stop codon. An

insertion is an addition of at least one base within the DNA. A deletion is a loss of at least one

base in the DNA sequence. Sickle cell anemia could also be considered a point mutation since

only a single base is changed which codes for a different amino acid.

8.The answer is A. There is no coding sequence between the TATA box (the promoter region where

RNA polymerase binds) and the transcription initiation site, so the presence of a TAG in the DNA

will not affect protein synthesis. In fact, the RNA corresponding to this sequence (UAG) will not

even be synthesized because the transcription initiation site occurs after this sequence in the DNA.

The introns and exons are not affected by this mutation, and splicing will occur normally. Thus, the

mRNA produced would be of normal size and would produce a normal-sized protein.

9.The answer is B. All tRNA molecules have the nucleotides CCA added to the 3′-end of the tRNA

after it has been transcribed. The ribose on the 3′-terminal adenine is the one that accepts amino

acids to form aminoacyl-tRNA. The absence of this A residue would prevent the charging of the

tRNA molecules, which would lead to cessation of all protein synthesis. It would require several

generations of growth such that any functional enzyme produced would have been

degraded after

that many generations. Spliceosomes are eukaryotic-specific because bacterial genes do not

contain introns that need to be spliced from the initial transcript. There are multiple aminoacyltRNA synthetases, and lacking one would lead to incomplete protein synthesis (protein synthesis

would stop when this particular charged tRNA needed to be brought to the ribosome), but some

protein synthesis would occur. Prokaryotic RNA is not capped; nor are poly(A) tails added to

each mRNA, as in eukaryotic cells. Prokaryotes do not contain a nucleus.

10. The answer is D. Diphtheria toxin catalyzes the ADP-ribosylation of eEF2, thereby inhibiting the

activity of this factor, which is required for the translocation step of protein synthesis. The toxin

does not affect RNA polymerase II, the peptidyltransferase activity of the large ribosomal subunit,

the initiation complex required for ribosome assembly, or the termination steps of protein

synthesis.16 Regulation of Gene Expression

For additional ancillary materials related to this chapter, please visit thePoint. Gene expression, the generation of a protein or RNA product from a particular gene, is controlled by

complex mechanisms. Normally, only a fraction of the genes in a cell are expressed at any time. Gene

expression is regulated differently in prokaryotes and eukaryotes.

Regulation of Gene Expression in Prokaryotes. In prokaryotes, gene expression is regulated mainly

by controlling the initiation of gene transcription. Sets of genes that encode proteins with related

functions are organized into operons, and each operon is under the control of a single promoter (or

regulatory region). Regulatory proteins called repressors bind to the promoter and inhibit the binding of

RNA polymerase (negative control), whereas activator proteins facilitate RNA polymerase binding

(positive control). Repressors are controlled by nutrients or their metabolites, classified as inducers or

corepressors. Regulation also may occur through attenuation of transcription. Eukaryotes: Regulation of Gene Expression at the Level of DNA. In eukaryotes, activation of a gene

requires changes in the state of chromatin (chromatin remodeling) that are facilitated by acetylation of

histones and methylation of bases. These changes in DNA determine which genes are available for

transcription.

Regulation of Eukaryotic Gene Transcription. Transcription of specific genes is regulated by proteins

(called specific transcription factors or transactivators) that bind to gene regulatory sequences

(called promoter-proximal elements, response elements, or enhancers) that activate or inhibit

assembly of the basal transcription complex and RNA polymerase at a TATA box or similar regulatory

element. These specific transcription factors, which may bind to DNA sequences some distance from the

promoter, interact with coactivators or corepressors that bind to components of the basal transcription

complex. These protein factors are said to work in “trans”; the DNA sequences to which they bind are

said to work in “cis.”

Other Sites for Regulation of Eukaryotic Gene Expression. Regulation also occurs during theprocessing of RNA, during RNA transport from the nucleus to the cytoplasm, and at the level of

translation in the cytoplasm. Regulation can occur simultaneously at multiple levels

for a specific gene,

and many factors act in concert to stimulate or inhibit expression of a gene. THE WAITING ROOM

Charles F., a 68-year-old man, complained of fatigue, loss of appetite, and a low-grade fever. An

open biopsy of a lymph node indicated the presence of non-Hodgkin lymphoma, follicular type.

Computed tomography and other noninvasive procedures showed a diffuse process with bone marrow

involvement. He is receiving multidrug chemotherapy with R-CHOP (rituximab, cyclophosphamide,

doxorubicin, vincristine, and prednisone).

Mannie W. is a 56-year-old man who complains of weight loss related to a decreased appetite and

increased fatigue. He notes discomfort in the left upper quadrant of his abdomen. On physical

examination, he is noted to be pale and to have ecchymoses (bruises) on his arms and legs. His spleen is

markedly enlarged.

Initial laboratory studies show a hemoglobin of 10.4 g/dL (normal = 13.5 to 17.5 g/dL) and a

leukocyte (white blood cell) count of 106,000 cells/mm3 (normal = 4,500 to 11,000 cells/mm3). The

majority of the leukocytes are granulocytes (white blood cells arising from the myeloid lineage), some of

which have an “immature” appearance. The percentage of lymphocytes in the peripheral blood is

decreased. A bone marrow aspiration and biopsy show the presence of an abnormal chromosome (the

Philadelphia chromosome) in dividing marrow cells.

Ann R., who has anorexia nervosa, has continued on an almost meat-free diet (see Chapters 1, 3, 9,

and 11). She now appears emaciated and pale. Her hemoglobin is 9.7 g/dL (normal = 12 to 16

g/dL), her hematocrit (volume of packed red cells) is 31% (reference range for women = 36% to 46%),

and her mean corpuscular volume (the average volume of a red cell) is 70 femtoliters (fL; 1 fL is 10−15 L)

(reference range = 80 to 100 fL). These values indicate an anemia that is microcytic (small red cells) and

hypochromic (light in color, indicating a reduced amount of hemoglobin per red cell). Her serum ferritin

(the cellular storage form of iron) was also subnormal. Her plasma level of transferrin (the iron transport

protein in plasma) is higher than normal, but its percentage saturation with iron is below normal. This

laboratory profile is consistent with changes that occur in an iron deficiency state.

Many of the drugs used by Charles F. inhibit the proliferation of cancer cells in various

ways. Doxorubicin (Adriamycin) is a large nonpolar molecule synthesized by fungi that

intercalates between DNA bases, inhibiting replication and transcription and forming DNA with

singleand double-strand breaks. Vincristine binds to tubulin and inhibits formation of the mitotic

spindle, thereby preventing cell division. Cyclophosphamide is an alkylating agent that damages

DNA by covalently attaching alkyl groups to DNA bases. Rituximab is an anti-CD20 antibody

which specifically targets B-cells (including the tumor cells) for destruction (a form of

immunotherapy). Prednisone is a steroid hormone; its effect on cancer cells is not exactlyunderstood, but it is given to help manage the side effects of the other agents.

The measurement of iron in the blood needs to account for iron associated with hemoglobin,

free iron, and iron bound to its carrier protein, transferrin. When a physician orders a

determination of serum iron, the order refers to ferric iron (Fe3+) bound to transferrin and not to

the ferrous iron (Fe2+) bound to circulating hemoglobin (which may be present in the plasma as a

result of occasional red-cell lysis). Measurement of ferric iron has been adapted to automated

spectrophotometric analysis. In most cases, the samples are acidified to remove the ferric ion

from transferrin. The iron is then reduced to the ferrous state by a reducing agent (such as ascorbic

acid), and the level of iron is determined by its binding to a dye that changes color when ferrous

ion binds to it.

The total iron-binding capacity (TIBC) is determined by adding ferric ion to the sample to

saturate all transferrin-binding sites in the sample. Excess iron (free iron, not bound to transferrin)

is precipitated by treatment with MgCO3, and the precipitate is removed by centrifugation. The

resulting soluble sample is then analyzed for Fe3+, as described previously. By comparing the

levels of Fe3+ bound both before and after saturation, one can determine what percentage of

transferrin contained bound iron. For a woman between the ages of 16 and 40 years, the

percentage saturation should be in the range of 20% to 50%. Ann R.’s iron saturation is below

this normal range, indicating an iron deficiency.

I. Gene Expression Is Regulated for Adaptation and Differentiation

Virtually all cells of an organism contain identical sets of genes. However, at any given time, only a small

number of the total genes in each cell are expressed (i.e., generate a protein or RNA product). The

remaining genes are inactive. Organisms gain several advantages by regulating the activity of their genes.

For example, both prokaryotic and eukaryotic cells adapt to changes in their environment by turning the

expression of genes on and off. Because the processes of RNA transcription and protein synthesis

consume a considerable amount of energy, cells conserve fuel by making proteins only when they are

needed.

In addition to regulating gene expression to adapt to environmental changes, eukaryotic organisms

alter expression of their genes during development. As a fertilized egg becomes a multicellular organism,

different kinds of proteins are synthesized in varying quantities. In humans, as the child progresses through

adolescence and then into adulthood, physical and physiologic changes result from variations in gene

expression and, therefore, of protein synthesis. Even after an organism has reached the adult stage,

regulation of gene expression enables certain cells to undergo differentiation to assume new functions.

II. Regulation of Gene Expression in Prokaryotes

Prokaryotes are single-celled organisms and therefore require less complex regulatory mechanisms than

the multicellular eukaryotes (Fig. 16.1). The most extensively studied prokaryote is the bacterium

Escherichia coli, an organism that thrives in the human colon, usually enjoying a symbiotic relationshipwith its host. Based on the size of its genome (4 × 106 base

pairs), E. coli should be capable of making

several thousand proteins. However, under normal growth conditions E. coli synthesizes only about 600

to 800 different proteins. Thus, many genes are inactive and E. coli will only synthesize those genes that

generate the proteins required for growth in that particular environment.

All E. coli cells of the same strain are morphologically similar and contain an identical circular

chromosome (see Fig. 16.1). As in other prokaryotes, DNA is not complexed with histones, no nuclear

envelope separates the genes from the contents of the cytoplasm, and gene transcripts do not contain

introns. In fact, as messenger RNA (mRNA) is being synthesized, ribosomes bind and begin to produce

proteins, so that transcription and translation occur simultaneously (known as coupled transcription–

translation). The mRNA molecules in E. coli have a very short half-life and are degraded within a few

minutes. mRNA molecules must be constantly generated from transcription to maintain synthesis of its

proteins. Thus, regulation of transcription, principally at the level of initiation, is sufficient to regulate the

level of proteins within the cell. A. Operons

The genes encoding proteins are called structural genes. In the bacterial genome, the structural genes for

proteins involved in performing a related function (such as the enzymes of a biosynthetic pathway) are

often grouped sequentially into units called operons (Fig. 16.2, and see Fig. 14.6). The genes in an operon

are coordinately expressed; that is, they are either all turned on or all turned off. When an operon is

expressed, all of its genes are transcribed (refer to Chapter 14, Section III). A single polycistronic mRNA

is produced that codes for all of the proteins of the operon. This polycistronic mRNA contains multiplesets of start and stop codons that allow several different proteins to be produced from this single

transcript at the translational level. Transcription of the genes in an operon is regulated by the promoter,

which is located in the operon at the 5′-end, upstream from the structural genes. B. Regulation of RNA Polymerase Binding by Repressors

In bacteria, the principal means of regulating gene transcription is through repressors, which are

regulatory proteins that prevent the binding of RNA polymerase to the promoter and thus act on initiation

of transcription (Fig. 16.3). In general, regulatory mechanisms such as repressors, which work through

inhibition of gene transcription, are referred to as negative control, and mechanisms that work through

stimulation of gene transcription are called positive control.

The repressor is encoded by a regulatory gene (see Fig. 16.3). Although this gene is considered part

of the operon, it is not always located near the remainder of the operon. Its product, the repressor protein,

diffuses to the promoter and binds to a region of the operon called the operator. The operator is located

within the promoter or near its 3′-end, just upstream from the transcription start point. When a repressor is

bound to the operator, the operon is not transcribed because the repressor protein either physically blocks

the binding of RNA polymerase to the promoter or prevents the RNA polymerase from initiatingtranscription. Two regulatory mechanisms work through controlling repressors: induction (an inducer

inactivates the repressor) and repression (a corepressor is required to activate the repressor).

1. Inducers

Induction involves a small molecule, known as an inducer, which stimulates expression of the operon by

binding to the repressor and changing its conformation so that it can no longer bind to the operator (Fig.

16.4). The inducer is either a nutrient or a metabolite of the nutrient. In the presence of the inducer, RNA

polymerase can therefore bind to the promoter and transcribe the operon. The key to this mechanism is

that in the absence of the inducer, the repressor is active, transcription is repressed, and the genes of the

operon are not expressed.

Consider, for example, induction of the lac operon of E. coli by lactose (Fig. 16.5). The enzymes for

metabolizing glucose by glycolysis are produced constitutively; that is, they are constantly being made. If

the milk sugar lactose is available, the cells adapt and begin to produce the three additional enzymes

required for lactose metabolism, which are encoded by the lac operon. A metabolite of lactose

(allolactose) serves as an inducer, binding to the repressor and inactivating it. Because the inactive

repressor no longer binds to the operator, RNA polymerase can bind to the promoter and transcribe the

structural genes of the lac operon, producing a polycistronic mRNA that encodes for the three additional

proteins. However, the presence of glucose can prevent activation of the lac operon (see “Stimulation of

RNA Polymerase Binding” in Section II.C). It is important to realize that the lac operon is expressed at

very low levels (basal levels) even in the absence of repressor. Thus, even in the absence of lactose, asmall amount of permease is present in the cellular membrane. Therefore, when lactose does become

available in the environment, a few molecules of lactose are able to enter the cell and can be metabolized

to allolactose. The few molecules of allolactose produced are sufficient to induce the operon. As the

amount of permease increases, more lactose can be transported into the cell to be used as an energy

source.

2. Corepressors

In a regulatory model called repression, the repressor is inactive until a small molecule called a

corepressor (a nutrient or its metabolite) binds to the repressor, activating it (Fig. 16.6). The repressor–

corepressor complex then binds to the operator, preventing binding of RNA polymerase and gene

transcription. Consider, for example, the trp operon, which encodes the five enzymes required for the

synthesis of the amino acid tryptophan. When tryptophan is available, E. coli cells save energy by no

longer making these enzymes. Tryptophan is a corepressor that binds to the inactive repressor, causing it

to change conformation and bind to the operator, thereby inhibiting transcription of the operon. Thus, in

the repression model, the repressor is inactive without a corepressor; in the induction model, the

repressor is active unless an inducer is present.C. Stimulation of RNA Polymerase Binding

In addition to regulating transcription by means of repressors that inhibit RNA polymerase binding to

promoters (negative control), bacteria regulate transcription by means of activating proteins that bind to

the promoter and stimulate the binding of RNA polymerase (positive control). Transcription of the lac

operon, for example, can be induced by allolactose only if glucose is absent. The presence or absence of

glucose is communicated to the promoter by a regulatory protein named the cyclic adenosine

monophosphate (cAMP) receptor protein (CRP) (Fig. 16.7). This regulatory protein is also called a

catabolite activator protein (CAP). A decrease in glucose levels increases levels of the intracellular

second messenger cAMP by a mechanism that involves glucose transport into the bacteria. cAMP binds to

CRP and the cAMP–CRP complex binds to a regulatory region of the operon, stimulating binding of RNA

polymerase to the promoter and transcription. When glucose is present, cAMP levels decrease, CRP

assumes an inactive conformation that does not bind to the operon, and the recruitment of RNA

polymerase to the promoter is reduced, resulting in inhibition of transcription. Thus, the enzymes encoded

by the lac operon are not produced if cells have an adequate supply of glucose, even if lactose is present

at very high levels.D. Regulation of RNA Polymerase Binding by Sigma Factors E. coli has only one RNA polymerase. Sigma factors bind to this RNA polymerase, stimulating its binding

to certain sets of promoters, thus simultaneously activating transcription of several operons. The standard

sigma factor in E. coli is σ70, a protein with a molecular weight of 70,000 Da (see Chapter 14). Other

sigma factors also exist. For example, σ32 helps RNA polymerase recognize promoters for the different

operons that encode the heat-shock proteins. Thus, increased transcription of the genes for heat-shock

proteins, which prevent protein denaturation at high temperatures, occurs in response to elevated

temperatures.

E. Attenuation of Transcription

Some operons are regulated by a process that interrupts (attenuates) transcription after it has been

initiated (Fig. 16.8). For example, high levels of tryptophan attenuate transcription of the E. coli trp

operon as well as repress its transcription. As mRNA is being transcribed from the trp operon, ribosomes

bind and rapidly begin to translate the transcript. Near the 5′-end of the transcript are several codons for

tryptophan. Initially, high levels of tryptophan in the cell result in high levels of Trp-tRNATrp and rapid

translation of the transcript. However, rapid translation generates a hairpin loop in the mRNA that servesas a termination signal for RNA polymerase, and transcription terminates. Conversely, when tryptophan

levels are low, levels of Trp-tRNATrp are low, and ribosomes stall at codons for tryptophan. A different

hairpin loop forms in the mRNA that does not terminate transcription, and the complete mRNA is

transcribed. Attenuation requires coupled transcription and translation, so this mechanism is not

applicable to eukaryotic systems.

The tryptophan, histidine, leucine, phenylalanine, and threonine operons are regulated, in part, by

attenuation. Repressors and activators also act on the promoters of some of these operons, allowing the

levels of these amino acids to be very carefully and rapidly regulated. III. Regulation of Gene Expression in Eukaryotes

Multicellular eukaryotes are much more complex than single-celled prokaryotes. As the human embryo

develops into a multicellular organism, different sets of genes are turned on and different groups of