Chloramphenicol Acetyltransferase: Mechanism & Uses

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

Chloramphenicol acetyltransferase (CAT, EC 2.3.1.28) is a bacterial enzyme that transfers an acetyl group from acetyl coenzyme A to the 3-hydroxyl group of the antibiotic chloramphenicol, producing 3-acetyl-chloramphenicol and inactivating the drug. The enzyme is a trimeric protein that uses a His-Asp catalytic dyad and follows a random-order kinetic mechanism in which either substrate can bind first.

CAT matters for two separate reasons that students often blur together. In nature it is a resistance weapon. The cat genes that encode it travel on plasmids and transposons and let bacteria survive chloramphenicol concentrations that would otherwise stop protein synthesis [1][2][3]. In the laboratory it became one of the first widely used reporter genes, because a short, well-understood promoter can be fused to cat and its activity read out by a simple enzymatic assay. That dual identity, resistance marker and reporter, is why the enzyme still appears in cloning manuals, transfection protocols, and antibiotic resistance surveillance data decades after it was first characterized.

What CAT Is and What It Is Not

The name "CAT" is a genuine source of confusion because three unrelated things share the abbreviation. The table below separates them.

The catalase collision is common in insect and plant molecular biology, where CAT routinely means catalase, an antioxidant enzyme. In a bee larval study, for example, researchers used dual-luciferase assays to show that a microRNA repressed a catalase gene labeled CAT [4]. That CAT has nothing to do with antibiotic resistance. When you read a paper, check the EC number or the reaction, not the abbreviation.

The Chemistry of the Reaction

CAT catalyzes a single acetyl transfer:

acetyl-CoA + chloramphenicol → CoA + 3-acetyl-chloramphenicol

The acetyl group lands on the 3-hydroxyl of the chloramphenicol molecule. This is the critical detail. Chloramphenicol binds the bacterial 50S ribosomal subunit and blocks the peptidyl transferase reaction, which stops protein synthesis. Acetylation of the 3-hydroxyl sterically blocks that ribosomal binding, so the modified drug can no longer occupy its target and the antibiotic loses its inhibitory potency [1]. The acetylated product is not toxic to the bacterium in any meaningful way, which is why a single acetylation event is enough to confer resistance.

Two points about the chemistry deserve emphasis.

First, the acetyl donor is acetyl-CoA, the same central metabolite used throughout carbon metabolism. CAT does not consume ATP and does not require a metal cofactor. It is a straightforward acyltransferase.

Second, the enzyme is not perfectly specific. CATI from Escherichia coli can acetylate the C21 hydroxyl of certain C21 hydroxysteroids, and whole-cell biocatalysts expressing catI convert those steroids regioselectively to 21-acetoxy derivatives [5]. This promiscuity is a useful reminder that "substrate specificity" in enzymology is a matter of degree. It also has environmental consequences. Because chloramphenicol has stereocenters, CAT can act stereoselectively, and the orientation around the first stereocenter influences which isomer is modified, which matters for how the drug and its metabolites behave in wastewater [6].

Enzyme Structure and the Catalytic Dyad

The trimer is the functional unit

CAT is active as a trimer, a three-subunit assembly. Each monomer contributes to a shared active site architecture, and the trimer is what creates the substrate-binding pocket. Structural work on CATIII co-crystallized with substrate analogs showed that one active site of the trimer had clear electron density for both acetyl-CoA and chloramphenicol while the other sites had weaker density for the cofactor [7]. That asymmetry is a real feature of the enzyme, not a crystallographic artifact, and it hints at cooperativity between subunits.

Native ion mobility spectrometry-mass spectrometry experiments on E. coli CATI support this picture. The enzyme readily binds multiple chloramphenicol molecules without large changes to its gas-phase structure or stability, and binding of subsequent ligands shows negative cooperativity [8]. The same study found that CATI binds a non-hydrolyzable acetyl-CoA analog more tightly than it binds chloramphenicol, and that chloramphenicol dissociates from the ternary complex at low collision energies while the cofactor stays bound even as the protein unfolds [8].

The His-Asp dyad

The catalytic machinery is a histidine-aspartate pair. In the accepted mechanism, the aspartate residue positions and polarizes the histidine, and the histidine acts as a general base that abstracts a proton from the 3-hydroxyl of chloramphenicol. The resulting alkoxide is a much better nucleophile and attacks the thioester carbonyl of acetyl-CoA, forming a transient tetrahedral intermediate. Collapse of that intermediate releases CoA and the acetylated drug.

Mutagenesis supports this model. In CATI, pentapeptide insertions that disrupt secondary structure elements containing active-site residues abolish chloramphenicol resistance in E. coli, while insertions in surface loops have little effect [9]. That pattern is exactly what you expect if the active site is a rigid, precisely positioned pocket rather than a flexible groove.

Random-order kinetics

CAT does not require acetyl-CoA to bind before chloramphenicol or vice versa. Either substrate can occupy the active site first, which makes the mechanism random-order rather than ordered. This is consistent with the structural data: chloramphenicol and acetyl-CoA binding sites overlap in some CAT variants, and each ligand can be accommodated independently [10][8].

The cat Gene Families

CAT enzymes are not one protein. They fall into several families with distinct sequences and folds, and the naming can be confusing.

• Type A (CATI): The classical, best-studied family. Broad clinical significance in Gram-negative bacteria [9].
• Type B (CatB): Adopts a hexapeptide repeat fold that is structurally distinct from Type A. CatB proteins from Pseudomonas aeruginosa, Vibrio cholerae, and Vibrio vulnificus share this fold, and a Type B CAT from Elizabethkingia anophelis also uses it [11].
• Type C (CATC): An intrinsic family found in Vibrio parahaemolyticus and related species. The catC genes sit in conserved genomic regions and are considered intrinsic to those species rather than recently acquired [12].

The distinction between acquired and intrinsic resistance matters. Acquired cat genes sit on mobile elements and can jump between species. Intrinsic cat genes are part of the chromosome and are inherited vertically. Both contribute to the resistance problem, but they spread differently.

Structural work continues to expand the family. The first high-resolution crystal structure of a Staphylococcus aureus CAT (saCAT1) revealed the active-site architecture and the basis for substrate recognition, with a Km of 16.9 µM against chloramphenicol [1]. That is a tight interaction, and it explains why the enzyme is efficient at low drug concentrations.

CAT as an Antibiotic Resistance Marker

In the bacterial world, cat is a resistance gene. It appears in a remarkable range of pathogens and environments.

In Klebsiella pneumoniae clinical isolates, the catB3 gene was found on a plasmid carrying blaOXA-1 and aac(6')-Ib-cr, part of a multidrug resistance cassette [3]. In Staphylococcus epidermidis, cat genes were identified in plasmid-like sequences from river isolates, identical to genes carried by Enterococcus faecium plasmids and associated with insertion sequence 6 family transposases [2]. In Shigella flexneri from Chilean children with acute diarrhea, all chloramphenicol-resistant strains carried a chromosomally located cat gene [13]. In Pseudomonas aeruginosa from hydrocarbon-contaminated soil, a catB7 gene was identified in every strain analyzed [14]. In Bacillus licheniformis and Bacillus paralicheniformis, a putative cat gene is present in the chromosome of all strains, and strains encoding a truncated CAT protein are chloramphenicol-sensitive, which is strong evidence that the gene is functional and intrinsic [15].

CAT also behaves as a shared resource in microbial communities. Because the enzyme is secreted or released, it can reduce local antibiotic concentrations and protect susceptible neighboring bacteria, a form of cooperative resistance [16]. The cat gene is frequently found alongside mobile genetic elements, which makes it a good candidate for horizontal transfer between species [16][2].

CAT as a Reporter Gene

The reporter use of CAT comes from a simple logic. If you place a cat coding sequence under the control of a promoter you want to study, then the amount of CAT enzyme produced reflects that promoter's activity. You then measure CAT enzyme activity, not CAT protein abundance directly.

This was one of the workhorse reporter systems of the 1980s and 1990s, especially for eukaryotic transfection assays. It has largely been displaced by luciferase, GFP, and β-galactosidase for routine work, but it remains useful when you need an enzymatic readout with low background, when you are working with a system where fluorescent proteins are problematic, or when you want to detect a secreted product.

The classic assay

The traditional CAT assay uses radioactive chloramphenicol and thin-layer chromatography. The steps are:

1. Transfect cells with a plasmid carrying your promoter fused to cat.
2. Harvest and lyse the cells after an appropriate expression period.
3. Prepare the reaction by mixing cell lysate with ¹⁴C-labeled chloramphenicol and acetyl-CoA.
4. Incubate at 37 °C to allow acetylation.
5. Extract the reaction products with an organic solvent such as ethyl acetate.
6. Spot the extract onto a thin-layer chromatography plate.
7. Develop the plate in a solvent system that separates chloramphenicol from its acetylated forms.
8. Expose the plate to film or a phosphorimager.
9. Quantify the ratio of acetylated to unacetylated chloramphenicol.

The readout is a ratio, so it is internally controlled. More promoter activity means more acetylated product and a higher ratio.

Modern alternatives

Radioactive assays are inconvenient and generate hazardous waste, so several alternatives exist.

• ELISA-based detection: An enzyme-linked immunosorbent assay can detect CAT protein or activity using antibodies, avoiding radioactivity.
• Fluorescent substrates: Fluorescent acetyl-CoA analogs or fluorescent chloramphenicol derivatives allow real-time monitoring of the reaction in a plate reader.
• Non-radioactive enzymatic assays: These use spectrophotometric or fluorometric detection of CoA release or product formation.

The choice depends on sensitivity requirements, throughput, and whether you need endpoint or kinetic data.

Comparing CAT with Other Reporters

CAT's main advantages are its low background in most eukaryotic cells (there is no endogenous CAT activity) and its enzymatic amplification, which means one enzyme molecule can turn over many substrate molecules. Its main disadvantages are the historical reliance on radioactivity and the fact that it is slower and less sensitive than luciferase.

Workflow Overview

The diagram below shows the decision path from choosing CAT as a reporter through to interpreting the assay result.

flowchart TD
    A[Choose reporter gene] --> B{Need enzymatic readout}
    B -->|Yes| C[Fuse promoter to cat]
    B -->|No| D[Consider GFP or luciferase]
    C --> E[Transfect cells]
    E --> F[Lyse and harvest]
    F --> G[Add labeled chloramphenicol and acetyl-CoA]
    G --> H[Incubate]
    H --> I[Extract products]
    I --> J[Separate by chromatography]
    J --> K[Quantify acetylated fraction]
    K --> L[Compare to control promoter]

Common Mistakes and Limitations

Confusing CAT with catalase. The abbreviation is shared. Always check the EC number or the reaction when reading a paper.

Assuming CAT is only a resistance gene. It is both a resistance determinant and a laboratory reporter. The same coding sequence serves both purposes.

Forgetting that the assay measures activity, not protein. A CAT assay tells you how much active enzyme is present, not how much cat mRNA or protein was made. If you need transcript levels, use qPCR or Northern blot.

Ignoring the trimer. CAT is not active as a monomer. Mutations that disrupt subunit interfaces can abolish activity even if the active site itself is intact.

Overlooking substrate promiscuity. CAT can acetylate non-antibiotic substrates such as certain steroids [5]. This matters for biocatalysis applications and for interpreting unexpected assay results.

Treating all cat genes as interchangeable. Type A, B, and C families differ in sequence, fold, and resistance level. A primer set designed for one family may not detect another [12][11].

Assuming resistance is always plasmid-borne. Some cat genes are chromosomal and intrinsic [15][13]. The genetic context affects how the resistance spreads.

Using the classic assay without controls. The radioactive CAT assay requires a no-lysate blank and a positive control lysate to interpret the ratio correctly.

Quick Review

1. CAT (EC 2.3.1.28) transfers an acetyl group from acetyl-CoA to the 3-hydroxyl of chloramphenicol, inactivating the drug.
2. The enzyme is a trimer with a His-Asp catalytic dyad and a random-order mechanism.
3. cat genes come in at least three families (A, B, C) with different sequences and folds.
4. In nature, CAT confers antibiotic resistance and can act as a shared resource in microbial communities.
5. In the lab, CAT is a reporter gene used in eukaryotic transfection assays.
6. The classic assay uses ¹⁴C-chloramphenicol and thin-layer chromatography. Modern alternatives include ELISA and fluorescent substrates.
7. CAT has low background in eukaryotic cells but is slower and less sensitive than luciferase.

Frequently Asked Questions

What does chloramphenicol acetyltransferase do?

It acetylates the 3-hydroxyl group of chloramphenicol, which prevents the drug from binding the bacterial ribosome and thereby inactivates it.

Is CAT the same as catalase?

No. CAT in this context is chloramphenicol acetyltransferase (EC 2.3.1.28). Catalase (EC 1.11.1.6) is a different enzyme that decomposes hydrogen peroxide.

Why is CAT used as a reporter gene?

Because eukaryotic cells have no endogenous CAT activity, so the enzymatic readout has very low background, and because one enzyme molecule can turn over many substrate molecules, which amplifies the signal.

What is the classic CAT assay?

It mixes cell lysate with ¹⁴C-labeled chloramphenicol and acetyl-CoA, separates the products by thin-layer chromatography, and quantifies the ratio of acetylated to unacetylated drug.

Are there non-radioactive CAT assays?

Yes. ELISA-based detection and fluorescent substrates are common alternatives that avoid radioactivity.

Do all bacteria with cat genes have the same resistance level?

No. Naturally occurring variants of CATC confer diverse resistance levels, and enzyme kinetics differ between variants [12].

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4. lncRNA1386.1/novel-miR0032-5p Axis Targets CAT Gene to Modulate Redox and Immune Reponses of Apis cerana Larvae to Ascosphaera apis Infection.
5. Regioselective Acetylation of C21 Hydroxysteroids by the Bacterial Chloramphenicol Acetyltransferase I.
6. Stereoselective metabolism of chloramphenicol by bacteria isolated from wastewater, and the importance of stereochemistry in environmental risk assessments for antibiotics.
7. Structures of chloramphenicol acetyltransferase III and Escherichia coli β-ketoacylsynthase III co-crystallized with partially hydrolysed acetyl-oxa(dethia)CoA.
8. Gas-phase stability and thermodynamics of ligand-bound, binary complexes of chloramphenicol acetyltransferase reveal negative cooperativity.
9. Characterization of permissive and non-permissive peptide insertion sites in chloramphenicol acetyltransferase.
10. Chloramphenicol Binding Sites of Acinetobacter baumannii Chloramphenicol Acetyltransferase CatB8.
11. Structural characterization of a Type B chloramphenicol acetyltransferase from the emerging pathogen Elizabethkingia anophelis NUHP1.
12. A novel family of intrinsic chloramphenicol acetyltransferase CATC in Vibrio parahaemolyticus: Naturally occurring variants reveal diverse resistance levels against chloramphenicol.
13. [[Molecular characterization of resistance mechanisms to chloramphenicol in Shigella flexneri strains isolated from Chilean children with acute diarrhea].](https://pubmed.ncbi.nlm.nih.gov/12043369/)
14. Antibiotic Resistance and Virulence Determinants of Pseudomonas aeruginosa Isolates Cultured from Hydrocarbon-Contaminated Environmental Samples.
15. Putative antibiotic resistance genes present in extant Bacillus licheniformis and Bacillus paralicheniformis strains are probably intrinsic and part of the ancient resistome.
16. Cooperative antibiotic resistance in bacteria: beyond biofilms.