Gene Control

Structure of the lac operon. The lac operon consists of: a regulator gene (lacI, located upstream and not strictly part of the operon), a promoter (P), an operator (O), and three structural genes (lacZ, lacY, lacA) coding for β-galactosidase, lac permease and transacetylase respectively.

The regulator gene lacI is transcribed and translated constitutively (always on) to produce repressor protein. The repressor has two binding sites: one for the operator DNA, and one for allolactose (an isomer of lactose).

(a) In the ABSENCE of lactose — operon OFF.

1. The repressor protein binds tightly to the operator sequence.
2. The bound repressor physically blocks the path of RNA polymerase. Although RNA polymerase can still bind the promoter, it cannot move forward to transcribe the structural genes.
3. No mRNA is produced for lacZ, lacY, lacA. No β-galactosidase or other lac enzymes are made.

This makes sense biologically — E. coli does not waste energy or amino acids producing lactose-digesting enzymes when there is no lactose to digest.

(b) In the PRESENCE of lactose — operon ON.

1. A small amount of lactose enters the cell (some leaks through the membrane even without lac permease, and a small basal level of permease is always present). Inside the cell, some lactose is converted to allolactose (its isomer).

2. Allolactose binds the repressor protein at its lactose-binding site.

3. Binding causes a conformational change in the repressor: the DNA-binding domain is distorted so the repressor can no longer bind the operator.

4. The repressor dissociates from the operator.

5. With the operator now clear, RNA polymerase can move forward from the promoter and transcribe the three structural genes as a single mRNA (polycistronic — one mRNA codes for three proteins).

6. The mRNA is translated to produce β-galactosidase (hydrolyses lactose to glucose + galactose), lac permease (transports lactose into the cell, accelerating the response) and transacetylase (function less critical).

7. Lactose is broken down for use as a respiratory substrate.

Self-regulation (negative feedback). As lactose is consumed, its concentration falls; allolactose dissociates from the repressor; the repressor refolds, re-binds the operator, and transcription stops. The operon turns itself off when its substrate is exhausted.

The lac operon is described as an inducible operon because lactose (its substrate) induces (switches on) its own breakdown. This is an elegant example of how cells balance gene expression with environmental conditions.

An inducible operon is one that is normally switched OFF (transcription is blocked) but is switched ON when a specific signalling molecule — the inducer — is present.

For the lac operon, the inducer is lactose (specifically its isomer allolactose). When lactose is absent, the repressor protein binds the operator and blocks transcription. When lactose is present, allolactose binds the repressor, causes a conformational change, and the repressor releases the operator. Transcription of the structural genes (β-galactosidase, lac permease, transacetylase) can then proceed.

The operon is therefore induced (switched on) only when its substrate is available — a logical regulatory design, since producing lactose-digesting enzymes when there is no lactose to digest would be a waste of cellular resources. As lactose is consumed and its concentration falls, the repressor re-binds the operator and transcription stops; the system is self-regulating.

Contrast with repressible operons. A repressible operon (e.g. the trp operon for tryptophan biosynthesis) is normally ON and is switched OFF when the end product accumulates. Inducible operons control catabolic pathways (breaking down substrates); repressible operons control anabolic pathways (synthesising products). The lac operon's logic is 'make the enzyme only when the substrate is here'.

Oestrogen as a steroid hormone. Oestrogen is a steroid hormone — a small, lipid-soluble (hydrophobic) molecule. Unlike peptide hormones (which need surface receptors and second messengers), oestrogen can pass directly through the phospholipid bilayer of target cell plasma membranes by diffusion.

Binding to the intracellular oestrogen receptor. In the cytoplasm (or nucleus) of target cells, oestrogen binds with high specificity to an intracellular oestrogen receptor protein. Cells without this receptor cannot respond to oestrogen — this is how the same hormone, circulating throughout the body, affects only certain tissues (uterus, breasts, bone, certain brain regions).

Conformational change. Binding of oestrogen causes the receptor to undergo a conformational change — the protein folds into a slightly different shape. This exposes a DNA-binding domain that was previously hidden. The oestrogen-receptor complex also translocates into the nucleus (if it was in the cytoplasm).

Binding to DNA — oestrogen response elements. Inside the nucleus, the oestrogen-receptor complex binds to specific DNA sequences called oestrogen response elements (EREs). These EREs are short sequences typically located in the regulatory (promoter/enhancer) regions upstream of target genes. The DNA-binding domain of the receptor recognises the ERE sequence specifically.

Activation of transcription. Once bound to the ERE, the oestrogen-receptor complex acts as a transcription factor: it recruits RNA polymerase II and other transcription machinery to the promoter, switching on transcription of the target gene. The newly produced mRNA is exported to the cytoplasm and translated into protein.

Examples of target genes activated by oestrogen. Proteins involved in:

• Female reproductive cycle (e.g. proteins for uterine lining growth).
• Secondary sexual characteristics (breast development, fat distribution).
• Bone density (oestrogen promotes osteoblast activity).
• Cell division in certain tissues (relevant in some breast cancers).

Comparison with peptide hormones. Peptide hormones (e.g. insulin, glucagon, ADH) are water-soluble and cannot cross the cell membrane; they bind surface receptors and trigger second messenger cascades (e.g. cAMP) inside the cell. Their effects are usually rapid and brief. Steroid hormones like oestrogen act much more slowly (transcription and translation take hours) but their effects are long-lasting because newly synthesised proteins persist.

Homeobox genes are a family of genes that all contain a highly conserved DNA sequence — the homeobox — about 180 base pairs long. The homeobox codes for a 60-amino-acid protein domain called the homeodomain, which is a DNA-binding region.

Hox genes as transcription factors. Because of their homeodomain, Hox gene products are transcription factors: they bind specific regulatory DNA sequences upstream of target genes and switch transcription of those target genes ON or OFF. Each Hox protein typically regulates dozens or hundreds of downstream genes.

Role in body plan — master regulators. Hox genes act as master regulators of body plan development. During embryonic development, they are expressed in a precise spatial and temporal sequence along the anterior-posterior axis of the embryo. Their expression pattern specifies the identity of each body region — i.e. which body segment will become head, which thorax, which abdomen. Hox genes are typically clustered on the chromosome in the same order that their expression appears along the body axis (a phenomenon called colinearity).

Conservation across the animal kingdom. Hox genes are astonishingly conserved across animals. The Hox genes that specify head-to-tail patterning in a fruit fly have direct homologues that perform the same role in a mouse and in a human. Mammals have four clusters of Hox genes (Hox A, B, C, D) that arose by gene duplication from a single ancestral cluster. This conservation reveals the deep evolutionary origin of the bilateral body plan, dating back >500 million years.

Effects of mutation — homeotic transformations. Mutations in Hox genes can cause homeotic transformations — entire body parts develop in the wrong location.

• Antennapedia mutation in Drosophila: legs develop on the head where antennae should be.
• Bithorax mutation: a second pair of wings develops on what should be the third thoracic segment.

These dramatic mutants prove that a single Hox gene specifies regional identity for an entire body segment.

Beyond Hox — homeobox genes in plants and fungi too. Other homeobox-containing genes (the MADS-box genes in plants, for example) control flower development. The homeobox is one of the most evolutionarily ancient and widely deployed DNA-binding motifs known.

Wider significance. Hox genes show that complex development can be controlled by a relatively small number of master regulator genes — a network of transcription factors that progressively specifies regional identity. This is a unifying principle of developmental biology.

Operon organisation. Prokaryotes typically organise functionally related genes into operons — a single promoter and operator control several structural genes that are transcribed together onto a single (polycistronic) mRNA. Example: lac operon (lacZ, lacY, lacA on one mRNA). Eukaryotes generally have individual gene promoters — each gene has its own promoter and is transcribed onto its own (monocistronic) mRNA. Operons are very rare in eukaryotes.

Speed and reversibility. Prokaryotic control (e.g. the lac operon) responds within seconds to minutes to environmental change — the repressor can rapidly bind or release the operator. Eukaryotic control is generally slower (transcription factors must be transported, chromatin must be remodelled, mRNA must be processed) — minutes to hours — but the responses are often longer-lasting.

Complexity of regulation. Prokaryotic regulation typically involves one or a few regulatory proteins (e.g. one repressor for the lac operon). Eukaryotic regulation involves many transcription factors binding to multiple regulatory sequences (promoters, enhancers, silencers). A typical eukaryotic gene may be regulated by 10-50 different transcription factors that combine to determine when and where the gene is expressed.

Chromatin and DNA accessibility. Eukaryotic DNA is wrapped around histone proteins to form chromatin. Before transcription can occur, the chromatin must be 'opened' (e.g. by histone acetylation, which loosens DNA-histone interactions) and 'closed' chromatin can silence genes for the life of the cell. Prokaryotic DNA is naked (no histones in bacteria) — this entire layer of regulation is absent.

Post-transcriptional control. Eukaryotes have extensive post-transcriptional regulation: alternative splicing (one gene → many proteins), mRNA capping and polyadenylation, mRNA stability control by miRNAs and siRNAs, and translational regulation. The pre-mRNA is processed in the nucleus before export. Prokaryotes transcribe and translate simultaneously (the ribosome attaches to the mRNA as it is being made) — there is no separate processing step. Post-transcriptional control is therefore minimal in prokaryotes.

Summary of key differences.

Feature	Prokaryotes	Eukaryotes
Gene organisation	Operons (polycistronic)	Individual gene promoters (monocistronic)
Speed	Fast (seconds-minutes)	Slow (minutes-hours)
Regulators	Few (one repressor)	Many transcription factors
Chromatin	None	Major regulator (histones)
Splicing	None	Yes (alternative splicing)
miRNA / siRNA	None / few	Extensive

Eukaryotic gene control is therefore far more complex and finely tuned, enabling the precise developmental programmes needed to build a multicellular organism.