Cyclic AMP is synthesized from ATP by an enzyme called adenylate cyclase, and glucose inhibits the synthesis of cyclic AMP and stimulates its transport out of the cell. When the mRNA is translated, the three different coding sequences of the mRNA are read separately, making three different proteins (Protein 1, Protein 2, and Protein 3). When glucose levels are high, no cAMP is made. This allows RNA polymerase to move forward on the DNA and transcribe the operon. Why is lac operon so important in modern molecular biology? Requested URL: byjus.com/biology/lac-operon-regulation-gene-expression/, User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.0.0 Safari/537.36. Consits of three adjacent structural genes. When glucose is absent, cyclic AMP (cAMP) accumulates. The lac operon is considered an _______ operon because it is usually turned off (repressed), but can be turned on in the presence of the inducer allolactose. These techniques provide a biochemical defintion of the operator = binding site for repressor. Manage Settings The reaso, Posted 6 years ago. It gets in RNA polymerase' s way, preventing transcription. Most of these genes encode proteins, each with its own role in a process such as fuel metabolism, maintenance of cell structure, and defense against viruses. The protein product of a repressor gene is the ___________ which binds the operator to stop transcription. Where do the regulatory proteins come from? Thus, cells don't express all the genes in their genome all the time. For the lac operon, the binding site is a dyad with that sequence in both sides of the dyad. As in most cases of cis-regulatory sequences, these are sites on DNA that are required for regulation. It floats off the operator, clearing the way for RNA polymerase to transcribe the operon. These factors can recruit the core RNA polymerase to promoters with specific DNA sequences and initiate gene transcription. This can be monitored by the ability of the protein-DNA complex to bind to nitrocellulose (whereas a radiolabeled mutant operator DNA fragement, oc, plus repressor will not bind). (3)tend to be distributed symmetrically around the dyad axis (+11). The viral nucleic acid is most likely. Early insights into mechanisms of transcriptional regulation came from studies of E. coli by researchers Francois Jacob & Jacques Monod. The lac operon is an operon, or group of genes with a _________ promoter (transcribed as a single mRNA). At a particular temperature and A]0=2.80103M,| \mathrm { A } ] _ { 0 } = 2.80 \times 10 ^ { - 3 }\ \mathrm { M },A]0=2.80103M, concentration versus time data were collected for this reaction, and a plot of 1/[A] versus time resulted in a straight line with a slope value of +3.60102Lmol1s1.+ 3.60 \times 10 ^ { - 2 } \mathrm { L }\ \mathrm { mol } ^ { - 1 } \mathrm { s } ^ { - 1 }.+3.60102Lmol1s1. These, Posted 5 years ago. The genes in the operon encode proteins that allow the bacteria to use lactose as an energy source. When lactose is absent, the, Lower panel: With lactose. The trp operon is regulated by the trp repressor. What is the term for the gene that codes for a protein capable of repressing the operon? The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. An example of data being processed may be a unique identifier stored in a cookie. When lactose is present outside the cell, it crosses the cell membrane and acts as an inducer of the operon. Direct link to http://facebookid.khanacademy.org/1476580007's post Why is lac operon so impo, Posted 6 years ago. We and our partners use cookies to Store and/or access information on a device. Bound CAP helps RNA polymerase attach to the lac operon promoter. This strategy ensures that bacteria only turn on the lac operon and start using lactose after they have used up all of the preferred energy source (glucose). On the other hand, the proteins that bind to these cis-elements are called trans-regulators because (as diffusible molecules) they do not necessarily need to be encoded on the same piece of DNA as the genes they regulate. Some of our partners may process your data as a part of their legitimate business interest without asking for consent. Isolation and purification of the protein was greatly aided by use of mutant strain with up-promoter mutations for lacI, so that many more copies of the protein were present in each cell. In the presence of the substrate, when the enzymes are needed, the operon is induced or de-repressed. The molecule is called an, Other operons are usually "on," but can be turned "off" by a small molecule. The operator overlaps the start the site of transcription and the promoter. This will be explored in more detail in Chapter 16. The mechanisms for these will be considered separately. Replication of the genome of DNA viruses occurs in the __________, whereas replication of the genome of RNA viruses occurs in the __________. Figure Detail. To u, Posted 6 years ago. The lac operon in E. coli controls the gene expression of the enzymes that digest lactose in the cell. http://facebookid.khanacademy.org/1476580007. The drawbacks could maybe be the possible mutations? Only then does RNA polymerase bind to the promoter. 1. Choose all correct answers. Repressor & activator proteins. Hello, thank you for visiting my blog. The lac operon produces enzymes that allow the bacteria E. coli to metabolize lactose, it is in an inducable operon. lacAencodes b-galactoside transacetylase; the function of this enzymes in catabolism of lactose is not understood (at least by me). Lac repressor remains bound to the operator and prevents binding of RNA polymerase. The ______ blocks RNA polymerase by binding to the operator. This page titled 15: Positive and negative control of gene expression is shared under a not declared license and was authored, remixed, and/or curated by Ross Hardison. It normally _________ transcription of the operon, but stops acting as a repressor when lactose is present. Regulatory proteins often bind to small molecules, which can make the protein active or inactive by changing its ability to bind DNA. Note that in this usage, the terms are defined by the reponse to a small molecule. Lactose Operon: An Inducer Operon. Smaller genomes It has a dyad symmetry centered at +11. (c) the electromagnetic interaction The lac operon's most important parts are the three genes, lacA, lacY, and lacZ, along with the promoter . Legal. We and our partners use data for Personalised ads and content, ad and content measurement, audience insights and product development. In the case of positive control, the genes are expressed only when an active regulator protein, e.g. We tend to think of bacteria as simple. and methylation interference assays (methylation of which purines will prevent binding?). RNA polymerase can now transcribe the operon. This is a protein that represses (inhibits) transcription of the lac operon. When the repressor tetramer is bound to o, lacZYAis not transcribed and hence not expressed. French scientists Franois Jacob (1920-2013) and Jacques Monod at the Pasteur Institute were the first to show the organization of bacterial genes into operons, through their studies on the lac operon of E. coli.They found that in E. coli, all of the structural genes that encode enzymes needed to use lactose as an energy source lie next to each other in the lactose (or lac) operon under the . When there is an absence of lactose the transcription of the lac operon genes is blocked by a repressor protein (as there will be no use of operon's gene products). The lactose operon (lac operon) is an operon required for the transport and metabolism of lactose in E. coli and many other enteric bacteria.Although glucose is the preferred carbon source for most bacteria, the lac operon allows for the effective digestion of lactose when glucose is not available through the activity of beta-galactosidase. Note that the latter is a genetic definition of the operator, and it coincides with the biochemically-defined operator. This means only few CAP (which were bound with cyclic AMP) will be able to bind to DNA. A single mRNA transcript includes all three enzyme-coding sequences and is called polycistronic. The lac operon encodes three structural genes necessary to acquire and process the disaccharide lactose from the environment, breaking it down into the simple sugars glucose and galactose. At the top of the diagram, we see a bacterial cell with a circular bacterial chromosome inside it. The upsid, Posted 5 years ago. As it turns out, RNA polymerase alone does not bind very well to the. These sequences are binding sites for regulatory proteins that turn expression of the operon "up" or "down.". When the repressor binds to the operator, it prevents RNA polymerase from binding to the promoter and/or transcribing the operon. Direct link to Ivana - Science trainee's post Yes. The third type of gene regulation in prokaryotic cells occurs through inducible operons, which have proteins that bind to activate or repress transcription depending on the local environment and the needs of the cell.The lac operon is a typical inducible operon.As mentioned previously, E. coli is able to use other sugars as energy sources when glucose . Direct link to Noaamir17's post does the suppressor regul, Posted 3 years ago. What is the net resistance? In the case of catabolite repressible enzymes, binding of RNA polymerase in the promoter region of DNA occurs only if catabolite activator protein (CAP) also known as CRP (cAMP Receptor Protein) has bound first. In this case (and many other cases), the operator is a region of DNA that overlaps with or lies just downstream of the RNA polymerase binding site (promoter). The CAP is a dimer protein with a binding site for cAMP and DNA. Virus cycle occurs in cytoplasm Direct link to bart0241's post Positive gene regulation , Posted 4 years ago. The key contact points (see Figure 4.1.4. The ________ of an operon is the location where RNA polymerase binds, whereas the _________ acts as the on/off switch for transcription of the structural genes. These sugars, such as lactose and glucose, require different enzymes for their metabolism. Eukaryotes generally do not group genes together as operons (exception is C. elegans and a few other species). b. When bound, the lac repressor gets in RNA polymerase's way and keeps it from transcribing the operon. If the cell has plenty of the product already (in the presence of the product), the the enzymes catalyzing its synthesis are not needed, and the operon encoding them is repressed. They lead to strong transcription of the, Now that weve seen all the moving parts of the, Glucose present, lactose absent: No transcription of the, Glucose present, lactose present: Low-level transcription of the, Glucose absent, lactose absent: No transcription of the, Glucose absent, lactose present: Strong transcription of the, Posted 5 years ago. Loses Thus, the lac operon can only be transcribed at high levels when glucose is absent. Even thou, Posted 4 years ago. Like any other protein produced in an organism, they are encoded by genes in the bacterium's genome. Instead, it also includes the promoter and other regulatory sequences that regulate expression of the genes. 4. the lac operon is induced in the presence of lactose (through the action of a metabolic by-product allolactose). (b) For the lac operon to be expressed, there must be activation by cAMP-CAP as well as removal of the lac repressor from the operator. Start your trial now! What condition is this? 5'-AAATAAC-3' a. lacY is a permease that helps to transfer lactose into the cell. This will cause loose binding of RNA polymerase to the promoter region resulting low level of transcription. [2]cAMP synthesis is catalyzed by adenylate cyclase (product of the cyagene). This encodes an enzyme that splits lactose into monosaccharides (single-unit sugars) that can be fed into glycolysis. This pattern of regulation might make sense for a gene involved in cell division in skin cells. Which type of operon is typically in the "off" position until the appropriate substrate is present? arrow . It is a source of nutritional components, antioxidants, and essential oils, which benefit our health and promote the function of Microbeonline.com is an online guidebook on Microbiology, precisely speaking, Medical Microbiology. They are under control of a single promoter (site where RNA polymerase binds) and they are transcribed together to make a single mRNA that has contains sequences coding for all three genes. How much time is required for the concentration of A\mathrm { A }A to decrease to 7.00104M?7.00 \times 10 ^ { - 4 }\ \mathrm { M } ?7.00104M? If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked. This binds to the lac repressor and makes it change shape so it can no longer bind DNA. d. 5'-TTCCCGGGATA-3', What interactions affect protons in an atomic nucleus? Lactose enter into cell with Help of permease.but permease enzyme is produced by lactose? That is, it is in between the promoter and the genes of the operon. Most strains with a defective repressor (, c. Strains with repressor that is not able to interact with the inducer (, d. Deductions based on phenotypes of mutants. Prokaryotic DNA is arranged into systems called _____. The lac operon has an added level of control so that the operon remains inactive in the presence of glucose even if lactose also is present. But even the simplest bacterium has a complex task when it comes to gene regulation! Login . A major type of gene regulation that occurs in prokaryotic cells utilizes and occurs through inducible operons. Thus o+ is dominant to oc when o+ is in cisto lacZ+. This control, is due to the positive regulatory protein called Catabolite Activator Protein (CAP), which is essential for the expression of lactose metabolising enzymes. [1]Product of the capgene, also called crp(cAMP receptor protein). glucose. No tracking or performance measurement cookies were served with this page. Lower panel: High glucose. Direct link to gdouvi's post what is the evolutionary , Posted 5 years ago. This arrangement allows E. coli to leverage the energetic balance between glucose and lactose utilization. Hope that helps! a. Score: 4.6/5 (57 votes) . Inducible operons are turned on in reponse to a metabolite (a small molecule undergoing metabolism) that regulates the operon. In this scenario, you need an operon that can be used to regulate the transcription of genes encoding proteins that function in the metabolic pathway used to synthesize uracil from . When inducer is present (signalling the presence of lactose), it binds the repressor protein, thereby altering its conformation, decreasing its affinity for o, the operator. it is a homotetramer). Can you give a couple examples of rare eukaryotic operons? These are connected by a "hinge" region. b. CAP helps RNA polymerase bind to the promoter, resulting in high levels of transcription. CAP helps RNA polymerase bind to the promoter, permitting high levels of transcription. Short lengths of RNA called __________ have the ability to control the expression of certain genes. Transcription of the structural genes of the arg operon is inhibited when arginine. (3)The allele of othat is in cisto the active reporter gene (i.e., on the same chromosome as lacZ+ in this case) is the one whose phenotype is seen. Positive gene regulation controls the production of genes by turning them on while negative gene regulation controls the production of genes by turning them off. The LAC Operon Encodes Proteins Involved in Lactose Metabolism - CAP: recognized by an activator protein known as the catabolite activator protein (CAP) - Operator: a binding site for a repressor protein called the LAC repressor - Initiation -> elongation = release of the sigma factor. In this compound the b-galactosidic linkage is to a thiol, which is not an efficient substrate for b-galactosidase. Direct link to Jack S. Gilbert's post How can the cell know tha, Posted 5 years ago. In general, an operon will contain genes that function in the same process. E.coli is a prokaryote and is one of the most known and studied one, so it is easy to use it as an example. Direct link to marie's post are there still sigma fac, Posted 3 years ago. CAP isn't always active (able to bind DNA). professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. It transforms lactose into allolactose and also catalyzes the conversion of lactose to glucose and galactose. In this condition, no transcription of the lac operon occurs. Which viruses insert their DNA into the host genome increasing the risk of cancer? When CAP is bound to at CBS, RNA polymerase is better able to bind to the promoter and initiate transcription. lacI-dprevents binding to DNA, leads to constitutive expression. if there was a mutation in the operator the repressor protein will not bind to the operator if there is no lactose in the environment. When lactose is present, _______ binds to the lac repressor and makes it let go of the operator. In the absence of the substrate,there is no reason for the catabolic enzymes to be present, and the operon encoding them is repressed. The lac Operon Encodes Proteins Involved in Lactose Metabolism - lacl gene - Regulatory gene - Not part of . Binding of allolactose with repressor protein changes the shape of repressor protein so it can no longer binds to the operator region. Thus, the, These two events in combination the binding of the activator and the release of the repressor allow RNA polymerase to bind strongly to the promoter and give it a clear path for transcription. When lactose is available, some molecules will be converted to _______ inside the cell. The lacZ gene encodes for the production of the enzyme -galactosidase (B), lacY gene for the production of the enzyme permease (P) and the lacA gene is responsible for the production of the enzyme thiogalactoside transacetylase. Repressible operons are normally turned on in the cell. Ch 9- An introduction to Microbial Genetics, Chapter 7: Microbial Nutrition, Ecology, and, John David Jackson, Patricia Meglich, Robert Mathis, Sean Valentine. Even bacteria can be picky about what they eat. lacISprevents binding of inducer, leads to a noninducible phenotype. When the cellular concentration of Trp (or Trp-tRNAtrp) is high, the operon is not expressed, but when the levels are low, the operon is expressed. Regulation of genes for lactose utilization. In the following exercise, find an equation of the circle that satisfies the given conditions. Direct link to amconnel99's post Great question. The concentration of cAMP is inversely proportional to the abundance of glucose: when glucose concentrations are low, an enzyme called adenylate cyclase is able to produce cAMP from ATP. A lac operon is a stretch of DNA that contains the genes which are able to encode proteins used in lactose metabolism. single (-) strand RNA. Overview of operons, regulatory DNA sequences, & regulatory genes. This phenomenon is called catabolite repression. Biology questions and answers. 2. CAP binds the CAP binding site of the lac promoter to carry out negative control of operon gene transcription, whereas cAMP blocks the CAP binding site and thereby allows fine-tuning of the system. This shape change makes the activator able to bind its target DNA sequence and activate transcription. 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\newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\), 12.2: The Use of Mutants to Study the lac Operon, Mount Royal University & University of Calgary, lacI is an allosterically regulated repressor, CAP is an allosteric activator of the lac operon, source@http://opengenetics.net/open_genetics.html, status page at https://status.libretexts.org. It does so once lactose is broken down to create allolactose. Riboswitches exert effects on __________ whereas repressors and inducers exert effects on __________. Continue with Recommended Cookies. What are they? Attenuation, or dampening, of the trp operon was discovered by examining E. coli that . Allolactose (rearranged lactose) binds to the, Wellnot quite. In a merodiploid strain, in which one copy of the lac operon is on the chromosome and another copy is on an F' factor, one can test for dominance of one allele over another. The desired DNA-binding protein can then be isolated by affinity chromatography, using the binding site in DNA as the affinity ligand. In this condition, the basal level transcription of the lac operon occurs. This page titled 12.1: The lac Operon is shared under a CC BY-SA 3.0 license and was authored, remixed, and/or curated by Todd Nickle and Isabelle Barrette-Ng via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request. The other, catabolite activator protein (CAP), acts as a glucose sensor. ], [Are regulatory genes found in the operon they regulate? For the transcription of structural genes by the lac operon, two requirements must be met: Each of the regulatory proteins (CAP and lac repressor) responds to one environmental signal and communicates it to the lac genes. (4)are largely on one face of the DNA double helix. Also, cAMP levels are low because glucose levels are high, so CAP is inactive and cannot bind DNA. lac operon is regulated by the lac repressor and catabolite activator protein (CAP). Alternative schemes will allow one to identify sites at which methylation is either prevented or enhanced by the binding of the repressor. These monosaccharides are broken down to lactate (principally via glycolysis, producing ATP), and from lactate to CO2 (via the citric acid cycle), producing NADH, which feeds into the electron-transport chain to produce more ATP (oxidative phosphorylation). True or false: Viral genomes are always composed of either single or double stranded DNA. CAP binds to a region of DNA just before the lac operon promoter and helps RNA polymerase attach to the promoter, driving ______ levels of transcription. A repressor protein binds the operator (control) region upstream of the operon preventing transcription.