All Labs

Gene Regulation Lab

Investigate how the lac operon is controlled by lactose and glucose signals. Build a regulatory logic table, run dose-response experiments to characterize the Hill equation, and generate a complete lab report with collected data.

Guided Experiment: lac Operon Logic Table

What combination of lactose and glucose do you predict will produce the highest expression of the lac operon? Why?

Write your hypothesis in the Lab Report panel, then click Next.

lac Operon Diagram

CAPCAPPromoterOperatorlacIlacZlacYlacARepressor: BoundCAP: ActiveTranscription: OFF

Controls

Lactose Present
Glucose Present

Results

Condition: Lac - / Glu -\text{Condition: Lac - / Glu -}
Repressor
Bound (blocking)
CAP-cAMP
Active
Transcription Rate
0.010
Fold Change
1.0x

Expression Comparison

Data Table

(0 rows)
#TrialOperon/GeneInducer LevelRepressorTxn Rate (rel)mRNA (rel)Protein (rel)Fold Change
0 / 500
0 / 500
0 / 500

Reference Guide

The lac Operon

The lac operon is a set of three genes (lacZ, lacY, lacA) in E. coli that encode enzymes for lactose metabolism. It is one of the best-studied examples of gene regulation.

Negative regulation by the lacI repressor protein prevents transcription by binding the operator when lactose is absent.

Positive regulation by CAP-cAMP enhances RNA polymerase binding when glucose is low.

Catabolite Repression

E. coli preferentially uses glucose over lactose. When glucose is present, cAMP levels drop and CAP cannot activate the lac promoter.

This creates an AND logic gate for maximum expression: both lactose must be present (to release repressor) AND glucose must be absent (for CAP activation).

Lac+∧Glu−⇒Max expression\text{Lac}^{+} \wedge \text{Glu}^{-} \Rightarrow \text{Max expression}

Hill Equation

The dose-response relationship between inducer concentration and gene expression is modeled by the Hill equation.

θ=[L]nKdn+[L]n\theta = \frac{[L]^n}{K_d^n + [L]^n}

The dissociation constant Kd is the concentration at which expression reaches 50% of maximum. The Hill coefficient n reflects cooperativity.

Measuring Gene Expression

In real experiments, lac operon expression is measured using the lacZ product, beta-galactosidase, which cleaves the chromogenic substrate ONPG to produce a yellow product.

Fold change compares expression under experimental conditions to a basal (uninduced) control.

Fold Change=ExpressioninducedExpressionbasal\text{Fold Change} = \frac{\text{Expression}_{\text{induced}}}{\text{Expression}_{\text{basal}}}

A fold change greater than 2 is typically considered biologically significant. The lac operon can achieve over 1000-fold induction in vivo.