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Unit 5: Microbiology The Genetics of Viruses and Bacteria PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Overview: Microbial Model Systems • Viruses called bacteriophages – Can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli Figure 18.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 0.5 m • E. coli and its viruses – Are called model systems because of their frequent use by researchers in studies that reveal broad biological principles • Beyond their value as model systems – Viruses and bacteria have unique genetic mechanisms that are interesting in their own right Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Recall that bacteria are prokaryotes – With cells much smaller and more simply organized than those of eukaryotes • Viruses – Are smaller and simpler still Virus Bacterium Animal cell Animal cell nucleus 0.25 m Figure 18.2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 18.1: A virus has a genome but can reproduce only within a host cell • Scientists were able to detect viruses indirectly – Long before they were actually able to see them Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Discovery of Viruses: Scientific Inquiry • Tobacco mosaic disease – Stunts the growth of tobacco plants and gives their leaves a mosaic coloration Figure 18.3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In the late 1800s – Researchers hypothesized that a particle smaller than bacteria caused tobacco mosaic disease • In 1935, Wendell Stanley – Confirmed this hypothesis when he crystallized the infectious particle, now known as tobacco mosaic virus (TMV) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Structure of Viruses • Viruses – Are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viral Genomes • Viral genomes may consist of – Double- or single-stranded DNA – Double- or single-stranded RNA Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Capsids and Envelopes • A capsid – Is the protein shell that encloses the viral genome – Can have various structures Capsomere of capsid RNA Capsomere DNA Glycoprotein 70–90 nm (diameter) 18 250 mm 20 nm 50 nm Figure 18.4a, b (a) Tobacco mosaic virus (b) Adenoviruses Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Some viruses have envelopes – Which are membranous coverings derived from the membrane of the host cell Membranous envelope Capsid RNA Glycoprotein 80–200 nm (diameter) Figure 18.4c 50 nm (c) Influenza viruses Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Bacteriophages, also called phages – Have the most complex capsids found among viruses Head Tail sheath DNA Tail fiber 80 225 nm Figure 18.4d Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 50 nm (d) Bacteriophage T4 General Features of Viral Reproductive Cycles • Viruses are obligate intracellular parasites – They can reproduce only within a host cell • Each virus has a host range – A limited number of host cells that it can infect Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Viruses use enzymes, ribosomes, and small molecules of host cells – To synthesize progeny viruses Entry into cell and uncoating of DNA DNA Capsid VIRUS Transcription Replication HOST CELL Viral DNA mRNA Viral DNA Capsid proteins Self-assembly of new virus particles and their exit from cell Figure 18.5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reproductive Cycles of Phages • Phages – Are the best understood of all viruses – Go through two alternative reproductive mechanisms: the lytic cycle and the lysogenic cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Lytic Cycle • The lytic cycle – Is a phage reproductive cycle that culminates in the death of the host – Produces new phages and digests the host’s cell wall, releasing the progeny viruses Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The lytic cycle of phage T4, a virulent phage 1 Attachment. The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell. 5 Release. The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles. 2 Entry of phage DNA and degradation of host DNA. The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell’s DNA is hydrolyzed. Phage assembly 4 Assembly. Three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms. Figure 18.6 Head Tails Tail fibers Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3 Synthesis of viral genomes and proteins. The phage DNA directs production of phage proteins and copies of the phage genome by host enzymes, using components within the cell. The Lysogenic Cycle • The lysogenic cycle – Replicates the phage genome without destroying the host • Temperate phages – Are capable of using both the lytic and lysogenic cycles of reproduction Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The lytic and lysogenic cycles of phage , a temperate phage Phage DNA The phage attaches to a host cell and injects its DNA. Phage DNA circularizes Phage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Bacterial chromosome Lytic cycle The cell lyses, releasing phages. Lysogenic cycle Certain factors determine whether Lytic cycle is induced Figure 18.7 Many cell divisions produce a large population of bacteria infected with the prophage. or New phage DNA and proteins are synthesized and assembled into phages. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lysogenic cycle is entered Prophage The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. Reproductive Cycles of Animal Viruses • The nature of the genome – Is the basis for the common classification of animal viruses Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Classes of animal viruses Table 18.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viral Envelopes • Many animal viruses – Have a membranous envelope • Viral glycoproteins on the envelope – Bind to specific receptor molecules on the surface of a host cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The reproductive cycle of an enveloped RNA virus 1 Glycoproteins on the viral envelope bind to specific receptor molecules (not shown) on the host cell, promoting viral entry into the cell. Capsid RNA Envelope (with glycoproteins) 2 Capsid and viral genome enter cell HOST CELL Viral genome (RNA) Template 5 Complementary RNA strands also function as mRNA, which is translated into both capsid proteins (in the cytosol) and glycoproteins for the viral envelope (in the ER). 3 The viral genome (red) functions as a template for synthesis of complementary RNA strands (pink) by a viral enzyme. mRNA Capsid proteins ER Glycoproteins Copy of genome (RNA) 4 New copies of viral genome RNA are made using complementary RNA strands as templates. 6 Vesicles transport envelope glycoproteins to the plasma membrane. 8 New virus 7 A capsid assembles Figure 18.8 around each viral genome molecule. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings RNA as Viral Genetic Material • The broadest variety of RNA genomes – Is found among the viruses that infect animals Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Retroviruses, such as HIV, use the enzyme reverse transcriptase – To copy their RNA genome into DNA, which can then be integrated into the host genome as a provirus Glycoprotein Viral envelope Capsid Reverse transcriptase Figure 18.9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings RNA (two identical strands) • The reproductive cycle of HIV, a retrovirus HIV Membrane of white blood cell 1 The virus fuses with the cell’s plasma membrane. The capsid proteins are removed, releasing the viral proteins and RNA. 2 Reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA. HOST CELL 3 Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first. Reverse transcriptase Viral RNA RNA-DNA hybrid 4 The double-stranded DNA is incorporated as a provirus into the cell’s DNA. 0.25 µm HIV entering a cell DNA NUCLEUS Chromosomal DNA RNA genome for the next viral generation Provirus mRNA 5 Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins. 6 The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). Figure 18.10 New HIV leaving a cell 9 New viruses bud off from the host cell. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 8 Capsids are assembled around viral genomes and reverse transcriptase molecules. 7 Vesicles transport the glycoproteins from the ER to the cell’s plasma membrane. Evolution of Viruses • Viruses do not really fit our definition of living organisms • Since viruses can reproduce only within cells – They probably evolved after the first cells appeared, perhaps packaged as fragments of cellular nucleic acid Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 18.2: Viruses, viroids, and prions are formidable pathogens in animals and plants • Diseases caused by viral infections – Affect humans, agricultural crops, and livestock worldwide Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viral Diseases in Animals • Viruses may damage or kill cells – By causing the release of hydrolytic enzymes from lysosomes • Some viruses cause infected cells – To produce toxins that lead to disease symptoms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Vaccines – Are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen – Can prevent certain viral illnesses Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Emerging Viruses • Emerging viruses – Are those that appear suddenly or suddenly come to the attention of medical scientists Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Severe acute respiratory syndrome (SARS) – Recently appeared in China (b) The SARS-causing agent is a coronavirus (a) Young ballet students in Hong Kong like this one (colorized TEM), so named for the wear face masks to protect themselves “corona” of glycoprotein spikes protruding from from the virus causing SARS. the envelope. Figure 18.11 A, B Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Outbreaks of “new” viral diseases in humans – Are usually caused by existing viruses that expand their host territory Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viral Diseases in Plants • More than 2,000 types of viral diseases of plants are known • Common symptoms of viral infection include – Spots on leaves and fruits, stunted growth, and damaged flowers or roots Figure 18.12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Plant viruses spread disease in two major modes – Horizontal transmission, entering through damaged cell walls – Vertical transmission, inheriting the virus from a parent Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viroids and Prions: The Simplest Infectious Agents • Viroids – Are circular RNA molecules that infect plants and disrupt their growth Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Prions – Are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals – Propagate by converting normal proteins into the prion version Prion Original prion Many prions Normal protein New prion Figure 18.13 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 18.3: Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria • Bacteria allow researchers – To investigate molecular genetics in the simplest true organisms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Bacterial Genome and Its Replication • The bacterial chromosome – Is usually a circular DNA molecule with few associated proteins • In addition to the chromosome – Many bacteria have plasmids, smaller circular DNA molecules that can replicate independently of the bacterial chromosome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Bacterial cells divide by binary fission – Which is preceded by replication of the bacterial chromosome Replication fork Origin of replication Termination of replication Figure 18.14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutation and Genetic Recombination as Sources of Genetic Variation • Since bacteria can reproduce rapidly – New mutations can quickly increase a population’s genetic diversity Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Further genetic diversity – Can arise by recombination of the DNA from two different bacterial cells EXPERIMENT Researchers had two mutant strains, one that could make arginine but not tryptophan trp–) and one that could make tryptophan but not arginine (arg trp+). Each mutant strain and a mixture of both strains were grown in a liquid medium containing all the required amino acids. Samples from each liquid culture were spread on plates containing a solution of glucose and inorganic salts (minimal medium), solidified with agar. (arg+ Mixture Mutant strain arg+ trp– Figure 18.15 Mutant strain arg trp+ RESULTS Only the samples from the mixed culture, contained cells that gave rise to colonies on minimal medium, which lacks amino acids. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mixture Mutant strain arg+ trp– Mutant strain arg– trp+ No colonies (control) CONCLUSION Colonies grew No colonies (control) Because only cells that can make both arginine and tryptophan (arg+ trp+ cells) can grow into colonies on minimal medium, the lack of colonies on the two control plates showed that no further mutations had occurred restoring this ability to cells of the mutant strains. Thus, each cell from the mixture that formed a colony on the minimal medium must have acquired one or more genes from a cell of the other strain by genetic recombination. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mechanisms of Gene Transfer and Genetic Recombination in Bacteria • Three processes bring bacterial DNA from different individuals together – Transformation – Transduction – Conjugation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transformation • Transformation – Is the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transduction • In the process known as transduction – Phages carry bacterial genes from one host cell to another Phage DNA A+ B+ 1 Phage infects bacterial cell that has alleles A+ and B+ A+ B+ 2 Host DNA (brown) is fragmented, and phage DNA and proteins are made. This is the donor cell. Donor cell 3 A bacterial DNA fragment (in this case a fragment with the A+ allele) may be packaged in a phage capsid. A+ 4 Phage with the A+ allele from the donor cell infects a recipient A–B– cell, and crossing over (recombination) between donor DNA (brown) and recipient DNA (green) occurs at two places (dotted lines). Crossing over A+ A– B– Recipient cell 5 The genotype of the resulting recombinant cell (A+B–) Figure 18.16 differs from the genotypes of both the donor (A+B+) and the recipient (A–B–). Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A+ B– Recombinant cell Conjugation and Plasmids • Conjugation – Is the direct transfer of genetic material between bacterial cells that are temporarily joined Figure 18.17 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sex pilus 1 m The F Plasmid and Conjugation • Cells containing the F plasmid, designated F+ cells – Function as DNA donors during conjugation – Transfer plasmid DNA to an F recipient cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Conjugation and transfer of an F plasmid from an F+ donor to an F recipient F Plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F– cell Bacterial chromosome 1 A cell carrying an F plasmid (an F+ cell) can form a mating bridge with an F– cell and transfer its F plasmid. 2 A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins to move into the recipient cell. As transfer continues, the donor plasmid rotates (red arrow). F+ cell 3 DNA replication occurs in 4 both donor and recipient cells, using the single parental strands of the F plasmid as templates to synthesize complementary strands. Figure 18.18a Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The plasmid in the recipient cell circularizes. Transfer and replication result in a compete F plasmid in each cell. Thus, both cells are now F+. (a) Conjugation and transfer of an F plasmid from an F+ donor to an F– recipient • Chromosomal genes can be transferred during conjugation – When the donor cell’s F factor is integrated into the chromosome • A cell with the F factor built into its chromosome – Is called an Hfr cell • The F factor of an Hfr cell – Brings some chromosomal DNA along with it when it is transferred to an F– cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F– recipient, resulting in recombination Hfr cell F+ cell F factor The circular F plasmid in an F + cell can be integrated into the circular chromosome by a single crossover event (dotted line). 1 Hfr cell A+ B+ C+ The resulting cell is called an Hfr cell (for High frequency of recombination). D+ C+ B+ D+ 2 D+ C+ A+ B+ A+ D+ C+ B+ B+ A+ B– A+ A+ F– cell 3 B– C– A– B+ D– Since an Hfr cell has all the F-factor genes, it can form a mating bridge with an F– cell and transfer DNA. B– A– B+ A+ B– C– A– A+ D– 4 A single strand of the F factor breaks and begins to move through the bridge. DNA replication occurs in both donor and recipient cells, resulting in double-stranded DNA Temporary partial diploid 7 C– B– C– D– A– 5 The location and orientation of the F factor in the donor chromosome determine the sequence of gene transfer during conjugation. In this example, the transfer sequence for four genes is A-B-C-D. B– D– Two crossovers can result in the exchange of similar (homologous) genes between the transferred chromosome fragment (brown) and the recipient cell’s chromosome (green). Figure 18.18b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A+ B+ C– A– D– 6 C– A– D– The mating bridge usually breaks well before the entire chromosome and the rest of the F factor are transferred. Recombinant F– bacterium 8 The piece of DNA ending up outside the bacterial chromosome will eventually be degraded by the cell’s enzymes. The recipient cell now contains a new combination of genes but no F factor; it is a recombinant F – cell. (b) Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F– recipient, resulting in recombination R plasmids and Antibiotic Resistance • R plasmids – Confer resistance to various antibiotics Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transposition of Genetic Elements • Transposable elements – Can move around within a cell’s genome – Are often called “jumping genes” – Contribute to genetic shuffling in bacteria Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Insertion Sequences • An insertion sequence contains a single gene for transposase – An enzyme that catalyzes movement of the insertion sequence from one site to another within the genome Insertion sequence 3 A T C C G G T… A C C G G A T… 3 5 TAG G C CA… TG G C CTA… 5 Transposase gene Inverted Inverted repeat repeat (a) Insertion sequences, the simplest transposable elements in bacteria, contain a single gene that encodes transposase, which catalyzes movement within the genome. The inverted repeats are backward, upside-down versions of each other; only a portion is shown. The inverted repeat sequence varies from one type of insertion sequence to another. Figure 18.19a Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transposons • Bacterial transposons – Also move about within the bacterial genome – Have additional genes, such as those for antibiotic resistance Transposon Insertion sequence Antibiotic resistance gene Insertion sequence 5 5 3 3 Inverted repeats Transposase gene (b) Transposons contain one or more genes in addition to the transposase gene. In the transposon shown here, a gene for resistance to an antibiotic is located between twin insertion sequences. The gene for antibiotic resistance is carried along as part of the transposon when the transposon is inserted at a new site in the genome. Figure 18.19b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 18.4: Individual bacteria respond to environmental change by regulating their gene expression • E. coli, a type of bacteria that lives in the human colon – Can tune its metabolism to the changing environment and food sources Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • This metabolic control occurs on two levels – Adjusting the activity of metabolic enzymes already present – Regulating the genes encoding the metabolic enzymes (a) Regulation of enzyme activity Precursor Feedback inhibition (b) Regulation of enzyme production Enzyme 1 Gene 1 Enzyme 2 Gene 2 Regulation of gene expression Enzyme 3 Gene 3 – Enzyme 4 Gene 4 – Enzyme 5 Gene 5 Tryptophan Figure 18.20a, b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Operons: The Basic Concept • In bacteria, genes are often clustered into operons, composed of – An operator, an “on-off” switch – A promoter – Genes for metabolic enzymes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An operon – Is usually turned “on” – Can be switched off by a protein called a repressor Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The trp operon: regulated synthesis of repressible enzymes trp operon Promoter DNA Promoter Genes of operon trpD trpC trpE trpR trpB trpA Operator Regulatory gene mRNA 5 3 RNA polymerase Start codon Stop codon mRNA 5 E Protein Inactive repressor D C B A Polypeptides that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes. Figure 18.21a Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off. As tryptophan accumulates, it inhibits its own production by activating the repressor protein. Figure 18.21b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Repressible and Inducible Operons: Two Types of Negative Gene Regulation • In a repressible operon – Binding of a specific repressor protein to the operator shuts off transcription • In an inducible operon – Binding of an inducer to an innately inactive repressor inactivates the repressor and turns on transcription Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The lac operon: regulated synthesis of inducible enzymes Promoter Regulatory gene DNA Operator lacl lacZ 3 mRNA Protein No RNA made RNA polymerase 5 Active repressor (a) Lactose absent, repressor active, operon off. The lac repressor is innately active, and in the absence of lactose it switches off the operon by binding to the operator. Figure 18.22a Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings lac operon DNA lacl lacz 3 mRNA 5 lacA RNA polymerase mRNA 5' 5 mRNA -Galactosidase Protein Allolactose (inducer) lacY Permease Transacetylase Inactive repressor (b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced. Figure 18.22b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Inducible enzymes – Usually function in catabolic pathways • Repressible enzymes – Usually function in anabolic pathways Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Regulation of both the trp and lac operons – Involves the negative control of genes, because the operons are switched off by the active form of the repressor protein Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Positive Gene Regulation • Some operons are also subject to positive control – Via a stimulatory activator protein, such as catabolite activator protein (CAP) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In E. coli, when glucose, a preferred food source, is scarce – The lac operon is activated by the binding of a regulatory protein, catabolite activator protein Promoter (CAP) DNA lacl lacZ CAP-binding site cAMP Inactive CAP RNA Operator polymerase can bind Active and transcribe CAP Inactive lac repressor (a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized. If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces Figure 18.23a large amounts of mRNA for the lactose pathway. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • When glucose levels in an E. coli cell increase – CAP detaches from the lac operon, turning it off Promoter DNA lacl lacZ CAP-binding site Operator RNA polymerase can’t bind Inactive CAP Inactive lac repressor (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized. When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription. Figure 18.23b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings