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#lac operon

25 public questions tagged with this topic.

Microbial Physiology

What happens in catabolite repression?

Glucose represents most energetically efficient carbon source giving maximal ATP per bond and direct entry into glycolysis via phosphotransferase system without induction cost. Bacteria evolved catabolite repression as global control suppressing alternative substrate utilization when glucose present to conserve resources. In Escherichia coli mechanism involves drop in second messenger cAMP due to inhibition of adenylate cyclase CyaA by dephosphorylated EIIA-Glc during glucose transport and prevention of active CRP-cAMP complex formation that binds conserved DNA sequence upstream of lac, araBAD, mal operons encoding alternative carbon catabolic enzymes and transporters. Inducer exclusion also occurs via EIIA-Glc binding LacY lactose permease blocking import. Consequently alternative carbon pathways remain silent while glucose abundant; when glucose exhausted cAMP rises five- to ten-fold, CRP-cAMP binds DNA and alternative operons become inducible after brief protein synthesis lag producing diauxic growth pattern with two exponential phases. Glucose catabolism itself continues and is prioritized; sporulation is distinct Spo0A-controlled developmental program, amino acid interconversion reflects nitrogen control but textbook catabolite repression focuses on glucose-mediated repression of alternative carbon utilization to maximize growth rate and efficiency.

Ref: Prescott's Microbiology, 11th ed., Chapter 6: Catabolite Repression and Glucose Preference in Bacteria.

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Lac Operon

lacA gene encodes enzyme involved in

lacA encodes beta-galactoside transacetylase, 203-amino-acid enzyme transferring acetyl group from acetyl-CoA donor to C6 hydroxyl acceptor of thiogalactosides and non-metabolizable lactose analogs. Crystallographic analysis reveals left-handed beta-helix fold assembled as trimer with active sites at subunit interfaces. Physiological role appears detoxification rather than primary catabolism: acetylation prevents accumulation of disulfide-forming thiogalactosides or other potentially toxic beta-galactosides that could irreversibly modify cellular proteins or glutathione, promoting their efflux via other transporters. Deletion mutants remain fully viable on lactose because hydrolysis by beta-galactosidase suffices, indicating auxiliary protective role rather than central catabolic requirement.

Ref: ResearchGate Lac operon GeneAct – lacA thiogalactoside transacetylase involved in cellular detoxification not catabolism.

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Lac Operon

lacY gene product is

lacY gene product is lactose permease LacY, 417-residue member of major facilitator superfamily transporter folded into twelve transmembrane alpha helices forming central hydrophilic cavity accessible alternately outward and inward during cycle. It operates as galactoside-proton symporter driven entirely by proton motive force, accumulating lactose against concentration gradient for metabolic use. Protein functionality is distinct from soluble beta-galactosidase encoded by lacZ and transacetylase LacA. By concentrating intracellular beta-galactosides, LacY enables efficient generation of allolactose inducer and provides substrate for catalytic cleavage, creating autocatalytic positive feedback essential for bistable switching behavior of lac operon.

Ref: ScienceDirect Lactose Permease overview – LacY transmembrane symporter concentrates β-galactosides for hydrolysis and induction.

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Lac Operon

CAP binding site is located

CAP, also called CRP, binds as homodimer to 22-base pair inverted repeat containing TGTGA-N6-TCACA consensus motif located upstream of lac promoter, typically centered at position -61.5. When glucose scarce, adenylate cyclase synthesizes second messenger cAMP that binds N-terminal effector domains of CAP, allosterically enabling C-terminal helix-turn-helix domains to specifically recognize major groove sequences. Dimer introduces approximately ninety-degree bend in DNA, contacts alpha-carboxy-terminal domain of RNA polymerase via activating region 1, stabilizes closed promoter complex, and accelerates isomerization to transcriptionally competent open complex. Upstream position avoids overlapping operator, permitting simultaneous positive and negative regulation through distinct sites.

Ref: NCBI Bookshelf – Sigma; J Bacteriol 1997: CAP-cAMP binds 61.5 bp upstream lac promoter, Type I activation.

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Lac Operon

lacOc mutation shows phenotype

Oc mutation alters the 21-base pair palindromic operator sequence overlapping the lac transcription start, disrupting critical base-specific contacts required for LacI repressor tetramer binding. Without stable repressor-operator interaction and tetrameric looping with auxiliary operators, promoter becomes freely accessible to sigma70 RNA polymerase holoenzyme initiating transcription. Consequently messenger synthesis of lacZYA continues even when allolactose inducer is absent. Because operator functions exclusively through cis linkage to downstream genes, the chromosome bearing Oc displays constitutive inducer-independent expression, while wild-type copy elsewhere remains regulatable, distinguishing cis-dominant constitutive from trans-recessive repressor defects.

Ref: LibreTexts 6.1.1 Use of Mutants Study lac Operon: Oc operator mutation blocks LacI binding causing constitutive.

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Lac Operon

lacOc mutation affects

Operator-constitutive mutations lacOc affect cis regulatory DNA element rather than diffusible trans-acting protein factor. Point mutations or small deletions in symmetric operator palindrome, especially central base pairs contacted by LacI recognition helix and critical guanine residues, drastically reduce LacI-operator binding affinity even for wild-type tetrameric repressor produced normally. Resulting transcription becomes constitutive but only for structural genes linked in cis to mutated operator on same DNA molecule, showing partial diploid cis-dominance in merodiploid tests, contrasting trans effects of lacI alleles. Promoter -10/-35 elements and CAP binding site remain structurally intact and functional, isolating defect to operator regulatory function alone.

Ref: Alberts Fig 7-38 lacOc mutation affects operator; NCBI - lacOc operator constitutive cannot bind repressor cis-dominant

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