What is the Difference Between CMV and CAG Promoter

The main difference between CMV and CAG promoter is that the transgene expression of the CMV promoter is low, whereas the transgene expression of the CAG promoter is high. 

CMV and CAG promoters are used to produce therapeutic recombinant proteins in mammalian cell lines. They provide the most efficient expression.

Key Areas Covered

1. What is CMV Promoter
– Definition, Characteristics, Importance
2. What is CAG Promoter
– Definition, Characteristics, Importance
3. Similarities Between CMV and CAG Promoter
– Outline of Common Features
4. Difference Between CMV and CAG Promoter
– Comparison of Key Differences

Key Terms 

CAG Promoter, CMV Promoter

What is CMV Promoter

CMV (cytomegalovirus) promoter is one of the several types of promoters used to express therapeutic proteins in mammalian cell lines. CHO cell lines are the type of mammalian cell lines used in the expression of therapeutic proteins. They can perform proper protein folding, assembly, and post-translational modifications similarly to human cells. Other cell lines are also important in the production of therapeutic proteins. Also, a promoter is a region that occurs upstream of the transgene. It initiates the transcription of the transgene. However, the activity of the promoter can differ depending on their composition. In addition to that, it differs depending on the cis-acting present in the transgenic region. Enhancers are the common type of cis-acting elements present in eukaryotic cells.

Figure 1: CHO Cells 

Furthermore, episomal vectors are used in the transfection of CHO cells. Importantly, the mouse CMV promoter is stronger than the human CMV promoter. But CMV promoter is commonly used in the high level of production of therapeutic recombinant proteins in mammalian cells. The expression level decreases with increasing culture time due to transcriptional silencing by DNA methylation. Therefore, other promoters are investigated to achieve stable expression along with increased expression levels. They are strong promoters in mammalian origin, synthetic, and are combined with cis-acting elements. 

What is CAG Promoter

CAG promoter is a strong, hybrid promoter used frequently for high gene expression levels in mammalian cell lines. Besides natural promoters, hybrid and synthetic promoters are important in the higher and long-term expression of the transgene. Importantly, the CAG promoter is a hybrid of CMV major immediate‐early enhancer, fused to chicken beta-actin promoter. It drives transgene expression in different cell lines. Also, it effectively increases and maintains the expression level under low-temperature conditions. Additionally, the temperature sensitivity of the CAG promoter is low, and it provides the most efficient expression.

Figure 2: CMV Genome

Moreover, the CAG promoter is a robust, artificial construct of CMV enhancer fused to the chicken beta-actin promoter. Importantly, it increases the expression level under low-temperature conditions. Significantly, using low temperatures in expression experiments is an effective strategy to improve the transgene expression. 

Similarities Between CMV and CAG Promoter

• CMV and CAG promoters are two promoters used to increase the efficiency of the expression of therapeutic recombinant proteins in mammalian cell lines. 

Difference Between CMV and CAG Promoter

Definition

CMV promoter refers to the major immediate-early promoter and enhancer of the human cytomegalovirus. In contrast, the CAG promoter refers to a strong synthetic promoter frequently used to drive high levels of gene expression in mammalian expression vectors. 

Type of Promoter

CMV promoter is a natural promoter, while CAG promoter is a hybrid promoter of CMV major immediate‐early enhancer, fused to chicken beta-actin promoter. 

Expression

The expression of the CMV promoter is low, while the expression of the CAG promoter is high.

Temperature Sensitivity

The temperature sensitivity of the CMV promoter is high, while the temperature sensitivity of the CAG promoter is low.

Conclusion

In brief, CMV and CAG promoters are two promoters used to produce therapeutic proteins in mammalian cell lines. The CMV promoter is a natural promoter. The expression of the CMV promoter is low. Also, its temperature sensitivity is high. In comparison, the expression of the CAG promoter is high. But its temperature sensitivity is low. Importantly, the CAG promoter is a hybrid of CMV major immediate‐early enhancer, fused to chicken beta-actin promoter. Therefore, the main difference between CMV and CAG promoters is their expression level. 

References:

1. Wang, W., Jia, Yl., Li, Yc. et al.RETRACTED ARTICLE: Impact of different promoters, promoter mutation, and an enhancer on recombinant protein expression in CHO cells. Sci Rep 7, 10416 (2017). 
2. Dou Y, Lin Y, Wang TY, Wang XY, Jia YL, Zhao CP. The CAG promoter maintains high-level transgene expression in HEK293 cells. FEBS Open Bio. 2021 Jan;11(1):95-104. doi: 10.1002/2211-5463.13029. Epub 2020 Dec 3. PMID: 33155423; PMCID: PMC7780116.

Image Courtesy:

1. “Cho cells adherend2” By User:Alcibiades – Own work (Public Domain) via Commons Wikimedia
2. “Genom cytomegalowirusa” By licensee MDPI, Basel, Switzerland – Own Work (CC-BY 3.0) via Commons Wikimedia