Reporter gene assays are a cornerstone of molecular biology for studying gene regulation. A reporter gene encodes a protein whose activity is easily measured, and it is placed under the control of a promoter or regulatory element of interest.
Common Reporters
- Firefly luciferase (from Photinus pyralis): catalyzes the oxidation of D-luciferin, producing light at 560 nm. The signal is measured with a luminometer. It has a very high dynamic range (7–8 orders of magnitude) and virtually no background in mammalian cells.
- Renilla luciferase (from Renilla reniformis): uses coelenterazine as a substrate, emitting at 480 nm. It is commonly used as a control reporter for normalization in dual-luciferase assays.
- Green fluorescent protein (GFP): from Aequorea victoria, fluoresces at 509 nm when excited at 395 nm. Variants cover the entire visible spectrum (CFP, YFP, RFP, mCherry). Fluorescence can be measured by fluorometer, flow cytometer, or microscopy, enabling live-cell and single-cell analysis.
- β-Galactosidase (LacZ): cleaves X-gal to produce a blue precipitate (colorimetric) or ONPG to produce a yellow product (absorbance at 420 nm). Simple and inexpensive but less sensitive than luciferase.
- SEAP (secreted alkaline phosphatase): secreted into the culture medium, allowing repeated sampling from the same culture without lysing cells. Measured by a chemiluminescent substrate.
- CAT (chloramphenicol acetyltransferase): the classical reporter, detected by ELISA or radioactive assay. Now largely replaced by luciferase and GFP.
Dual-Luciferase Assays
Dual-luciferase assays use firefly and Renilla luciferase in the same sample. The test promoter drives firefly luciferase, and a constitutive promoter (e.g., SV40, TK) drives Renilla luciferase as an internal control. After measuring firefly luminescence, a reagent quenches it and activates Renilla luminescence. The ratio firefly/Renilla normalizes for transfection efficiency and cell number.
Applications
Reporter assays are used to:
- Map promoter and enhancer regions
- Study transcription factor activity
- Measure signaling pathway activation (e.g., NF-κB, CREB, STAT)
- Screen for compounds that modulate gene expression
- Monitor miRNA activity (3′ UTR reporters)
- Quantify transfection efficiency
