The lac operon (lactose operon) of E. coli is the classic example of gene regulation in prokaryotes. Proposed by Jacob and Monod (1961, Nobel Prize 1965). It controls the metabolism of lactose (a disaccharide of glucose + galactose). E. coli prefers glucose; when glucose is absent but lactose is present, the lac operon is switched on to produce enzymes for lactose utilisation. The operon contains: regulatory sequences (promoter, operator) and structural genes (z, y, a). The i gene (regulator) is separate but nearby. The entire system demonstrates elegant negative regulation — a repressor keeps the system OFF; inducer removes the repressor to turn ON.

From 5' to 3' on the chromosome: i gene (regulatory): encodes the lac repressor protein. Has its own promoter (Pi). Promoter (P): where RNA polymerase binds to initiate transcription of structural genes. Operator (O): short DNA sequence overlapping/adjacent to promoter. Site where repressor binds to block transcription. z gene (structural): encodes β-galactosidase. This enzyme primarily hydrolyses lactose → glucose + galactose. Also converts some lactose → allolactose (the true inducer). y gene (structural): encodes permease (also called galactoside permease). Increases permeability of E. coli membrane for β-galactosides → more lactose enters the cell. a gene (structural): encodes transacetylase. Transfers acetyl group to non-metabolisable β-galactosides → may help their excretion. All three structural genes are transcribed as one polycistronic mRNA.

Negative regulation = a repressor keeps the operon OFF by default. When glucose present, lactose absent: i gene constitutively expressed → repressor protein synthesised → repressor binds to operator (O) → sterically blocks RNA polymerase from transcribing z, y, a → no enzymes → lactose cannot be metabolised. When lactose present (even if glucose absent): lactose enters cell (small amount via basal level of permease) → some lactose converted to allolactose by β-galactosidase → allolactose is the TRUE INDUCER → allolactose binds repressor → changes repressor conformation → repressor cannot bind operator → repressor releases from operator → RNA polymerase can now transcribe z, y, a → β-galactosidase, permease, transacetylase produced → lactose metabolised. Key point: lactose itself is NOT the inducer — allolactose (the isomeric product of lactose) is the actual inducer.

The lac operon also has positive regulation. When glucose is present: glucose inhibits adenylate cyclase → low cAMP → CAP (catabolite activator protein) has no cAMP → CAP cannot bind → low transcription. When glucose is absent (catabolite repression lifted): adenylate cyclase active → high cAMP → cAMP binds CAP → CAP-cAMP complex binds to CAP site upstream of lac promoter → RNA polymerase binds MORE effectively → increased transcription. Glucose preference (catabolite repression): even if lactose is present, if glucose is ALSO present, the lac operon is only minimally expressed (cAMP low → CAP inactive → low transcription). Only when glucose is exhausted AND lactose is present does full lac operon expression occur. This ensures E. coli uses the preferred energy source (glucose) first.

Jacob and Monod used mutants to dissect the operon: Constitutive mutants: lac operon expressed even without lactose. Two types: Oc (operator constitutive): operator mutation → repressor cannot bind → always expressed. Ic (repressor constitutive): i gene mutation → repressor cannot bind allolactose → always represses. Wait — a constitutive mutant would always EXPRESS, not always repress. So Oc → always on (cis-dominant). Ic⁻ (negative repressor mutant) → repressor non-functional → always on (trans-dominant). Merodiploid analysis (partial diploid): used to determine cis vs trans effects. Operator mutations (Oc) are cis-dominant — affect only genes on same chromosome. Repressor mutations (Ic⁻) are trans-recessive — a normal i⁺ allele on other chromosome can supply functional repressor. This distinction proved operator is a cis-regulatory element, repressor acts in trans.

The trp operon (tryptophan operon) in E. coli is another important example of gene regulation. It controls tryptophan biosynthesis — the opposite type of regulation from lac operon. Lac operon: inducible — normally OFF, turned ON when substrate (lactose) is present. Trp operon: repressible — normally ON, turned OFF when product (tryptophan) accumulates. When tryptophan is scarce: aporepressor (from trpR gene) cannot bind operator → RNA polymerase transcribes trpE, trpD, trpC, trpB, trpA → tryptophan biosynthetic enzymes produced. When tryptophan is abundant: tryptophan acts as co-repressor → binds aporepressor → repressor-corepressor complex binds operator → blocks transcription → saves energy when product is available. Attenuation: additional regulatory mechanism in trp operon — premature termination of transcription when Trp is available (involves ribosome pausing and RNA secondary structure).

Prokaryotic: operons (polycistronic). Rapid response (transcription + translation coupled). Mainly transcriptional control. Simpler regulatory circuits. Eukaryotic: no operons (monocistronic mRNA). Transcription and translation SEPARATED in space (nucleus vs cytoplasm) and time. Multiple levels of regulation: transcriptional (enhancers, silencers, TFs), post-transcriptional (splicing, RNA stability), translational (initiation control), post-translational (protein modification, degradation). Enhancers: can act at great distances (100 kb+) from promoter via DNA looping. Chromatin remodelling required for transcription access. Hormone response elements: steroid hormones enter nucleus → bind nuclear receptors → regulate gene expression. No operon equivalent in eukaryotes — co-regulated genes not clustered (exception: histone genes, some developmental gene clusters like Hox genes).

Gene expression is controlled at multiple levels to allow precise responses. Transcriptional control: most important. Which genes are transcribed? Regulated by: promoter strength, transcription factors (activators and repressors), enhancers and silencers, chromatin accessibility (histone modifications). Post-transcriptional control: pre-mRNA processing: alternative splicing, alternative polyadenylation. mRNA stability: deadenylation → decapping → degradation. RNA editing (A→I in some mRNAs). mRNA export from nucleus. miRNA (microRNA): 21-22 nt non-coding RNA → pairs with 3'UTR of mRNA → blocks translation OR causes degradation. siRNA: 21 nt dsRNA → RISC complex → mRNA degradation. Translational control: availability of ribosomes, initiation factors, codons. Post-translational control: protein folding (chaperones), post-translational modifications (phosphorylation, ubiquitination → degradation by proteasome), protein localisation.