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I know that the CaMV 35S promoter is widely used for transgenic plants, where it acts as an enhancer element for constitutive overexpression.

I noticed that it is always used as a tandemly repeated enhancer with 4 copies.

Why 4 copies? Why not 1, 2, 3, 5, or 10?

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When the enhancer was first studied it was obviously in a single copy.

Later work found that tandem duplication (2 copies) increased expression levels greatly. This 2-copy construct was then used in some studies.

Here is a quote from a 2003 paper (Ma et al. 2003 Nature Reviews Plants):

The CaMV 35S promoter is now the most popular choice in DICOTYLEDONOUS PLANTS (dicots). It is a strong constitutive promoter that can be made even more active by duplicating the enhancer region.

By 2009 the 4X35S is showing up in publications but I have not had any luck tracking down its origin.

So in brief: the exact number is because of history. Earlier on they used fewer copies.

There are probably diminishing returns to enhancer strength with more copies, and fewer copies is not strong enough.

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