Concept explainers
Four independent
The strains have the following
Mutant A is lac-, but transcription of operon genes is induced by lactose.
Mutant B is lac- and has uninducible transcription of operon genes.
Mutant C is lac+ and has constitutive transcription of operon genes.
Mutant D is lac+ and has constitutive transcription of operon genes.
A microbiologist develops donor and recipient varieties of each mutant strain and crosses them with the results shown below. The table indicates whether inducible, constitutive, or noninducible transcription occurs, along with lac+ and lac- growth habit for each partial diploid. Assume each strain has a single mutation.
Use the information to identify which lac operon gene is mutated in each strain.
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Genetic Analysis: An Integrated Approach (2nd Edition)
- . Suppose you have six strains of E. coli. One is wildtype, and each of the other five has a single one of thefollowing mutations: lacZ−, lacY−, lacI−, oc, andlacIS. For each of these six strains, describe thephenotype you would observe using the following assays. [Notes: (1) IPTG is a colorless synthetic molecule that acts as an inducer of lac operon expressionbut cannot serve as a carbon source for bacterialgrowth because it cannot be cleaved byβ-galactosidase; (2) X-gal cannot serve as a carbonsource for growth; (3) E. coli requires active lactosepermease (the product of lacY) to allow lactose,X-gal, or IPTG into the cells.]a. Growth on medium in which the only carbonsource was lactose.b. Colony color in medium containing glycerol as theonly carbon source, X-gal, and IPTGarrow_forwardLet’s suppose you have isolated a mutant strain of E. coli in which the lac operon is constitutively expressed. In other words, the operon is turned on in the presence or absence of lactose. One possibility is that the mutation may block the transcription of the lacI gene, thereby preventing the synthesis of lac repressor. A second possibility is that the mutation could alter the sequence of the lac operon in a way that prevents the repressor protein from binding to the operator. How would you distinguish between these two possibilities?arrow_forwardAn E. coli cell acquires a mutation in the gene that encodes CAP. The mutation affects the DNA-binding domain of CAP and ablates the ability of CAP to bind to the CAP site. Under which condition will the genes of the lac operon be transcribed at high levels? Glucose absent, lactose absent Glucose absent, lactose present Glucose present, lactose absent Glucose present, lactose present Under any of these conditions, the lac operon will not be transcribed at high levelsarrow_forward
- For each of the E. coli strains containing lac operon alleles listed, indicate whether the strain is inducible, constitutive, or unable to express beta-galactosidase and permease. (P+ and P- are functional and nonfunctional promoters, respectively) I+ P+ o+ Z- Y+ / I+ P+ oc Z+ Y+ I+ P+ o+ Z+ Y+ / I- P+ oc Z+ Y- I+ P+ o+ Z- Y+ / I- P+ oc Z+ Y- I- P- o+ Z+ Y- / I+ P+ oc Z- Y+ IS P+ o+ Z+ Y+ / I- P+ o+ Z+ Y-arrow_forwardThe lac operon produces enzymes that allow the bacteria E. coli to metabolize lactose, it is in an inducable operon. When lactose is present, will the repressor protein attach or leave the operon? How would this effect the production of lactose producing enzymes?arrow_forwardLet’s suppose you have isolated a mutant strain of E. coli in which the lac operon is constitutively expressed. In other words, the operon is turned on in the presence or absence of lactose. One possibility is that the mutation mayblock the transcription of the lacI gene, thereby preventing the synthesis of lac repressor. A second possibility is that the mutation could alter the sequence of the lac operator in a way that prevents lac repressor from binding to the operator. How would you distinguish between these two possibilities?arrow_forward
- You have isolated different mutants (reg1 and reg2) causing constitutive expression of the emu operon (which has genes emu1 and emu2). One mutant contains a defect in a DNA-binding site, and the other has a loss-of-function defect in the gene encoding a protein that binds to the site. Is the DNA-binding protein a positive or negative regulator of gene expression?arrow_forwardIn the absence of lactose and the lacY structural gene contains a missense point mutation, the structural genes will: [ Select ] ["No longer be expressed", "Be constantly expressed", "Be deleted from the operon"] In the presence of lactose and the lacY structural gene contains a missense point mutation, the structural genes will: [ Select ] ["No longer be expressed", "Be constantly expressed", "Be deleted from the operon"] In the absence of lactose and the lacA structural gene contains a frameshift deletion mutation, the structural genes will: [ Select ] ["No longer be expressed", "Be constantly expressed but the protein will be truncated and possibly non-functional", "Be constantly expressed but the protein will be longer and possibly non-functional"] In the presence of lactose and the lacA structural gene contains a frameshift deletion mutation, the structural genes will: [ Select ] ["No longer be expressed", "Be constantly expressed but the protein will be truncated and possibly…arrow_forwardGive all possible genotypes of a lac operon that produces, or fails to produce, β-galactosidase and permease under the following conditions. Do not give partial-diploid genotypes. Lactose absent Lactose present β-Galactosidase Permease β-Galactosidase Permease a. − − + + b. − − − + c. − − + − d. + + + + e. − − − − f. + − + − g. − + − +arrow_forward
- How does the repressor protein contribute to selective expression of the structural genes Z, Y, and A in the lac operon? The repressor protein is only expressed when lactose is present. The repressor is always expressed. It blocks transcription when lactose is present. The repressor is always expressed. It allows transcription when lactose is absent. The repressor is always expressed. It blocks transcription when lactose is absent. The repressor protein is only expressed when lactose is absent.arrow_forwardFor the following questions, describe the expression levels of the structural genes in the Lac operon under the conditions described. In the absence of lactose, the structural genes will:[ Select ] ["No longer be expressed", "Be constantly expressed", "Be deleted from the operon"] In the presence of lactose, the structural genes will:[ Select ] ["No longer be expressed", "Be constantly expressed", "Be deleted from the operon"] In the absence of lactose and the promoter sequence was mutated such that the RNA polymerase can no longer bind, the structural genes will:[ Select ] ["No longer be expressed", "Be constantly expressed", "Be deleted from the operon"] In the presence of lactose and the promoter sequence was mutated such that the RNA polymerase can no longer bind, the structural genes will:[ Select ] ["No longer be expressed", "Be constantly expressed", "Be deleted from the operon"] In the absence of lactose and the lacI repressor gene is mutated such that the repressor protein…arrow_forwardIf glucose is not available, but lactose is available from the environment, what is the status of transcription of the lac operon genes? Explain your answer from both an evolutionary perspective and in terms of negative and positive regulation of the operon?arrow_forward
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