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What’s Plasmid Copy Quantity and Straightforward Methods to Manipulate it

Plasmid copy quantity can have an infinite impression in your yields, and typically having a excessive plasmid copy quantity is finest, however different occasions it may be detrimental.

Be part of us as we take a more in-depth take a look at plasmid copy quantity and look at methods this may be manipulated within the lab supplying you with flexibility in your work.

On the agenda:

  1. What precisely does plasmid copy quantity imply?
  2. Why is plasmid copy quantity vital?
  3. How can plasmid copy quantity be manipulated?

What’s Meant by Copy Quantity?

Plasmid copy quantity refers back to the common or anticipated variety of copies per host cell. Plasmids are both low, medium, or excessive copy quantity. Plasmids range broadly in copy quantity relying on three predominant components:

1) The ori and its constituents (e.g., ColE1 RNA I and RNA II).

2) The scale of the plasmid and its related insert (larger inserts and plasmids could also be replicated at a decrease quantity as they signify an amazing metabolic burden for the cell).

3) Tradition circumstances (i.e., components that affect the metabolic burden on the host).

QIAGEN has a useful desk with a number of at present accessible plasmids and their respective copy numbers. Whereas there are variations in the way in which plasmids are categorized in keeping with copy quantity, a really common rule of thumb is proven in Desk 1.

Desk 1. Common copy numbers for low, medium, and excessive copy quantity plasmids.

Typical variety of copies per bacterial cell

Low copy (e.g. pBR22 and derivatives)

Excessive copy (e.g. pUC18, pUC19 vectors)

500–700 copies per cell

Why is it Necessary to Know the Copy Quantity For Your Plasmid?

It’s essential to know which class your plasmid falls into earlier than beginning your experiment. If you realize you’re working with a low-copy quantity plasmid, you shouldn’t be too stunned with a low yield, and also you may resolve to arrange extra cultures.

Then again, for those who get a poor yield from a excessive copy plasmid, you realize it’s essential do some troubleshooting, assuming your insert isn’t too giant!

A bonus of a excessive plasmid copy quantity is the higher stability of the plasmid when random partitioning (i.e., partitioning of plasmids into daughter cells) happens at cell division. Nonetheless, a excessive variety of plasmids also can lead to decrease yields.

Let’s take a look at a number of instances the place you really want to contemplate copy quantity in your experiments:

When is a Excessive-Copy Quantity Good?

There are a number of conditions the place you desire a excessive copy quantity plasmid, together with:

When is a Low Copy Quantity Good?

You may suppose {that a} excessive copy quantity is at all times finest, however some conditions are higher suited to a low copy quantity.

  • Expressing a poisonous product
    Let’s say you need to research a fungal protein for its antibacterial properties, and also you need to categorical it in micro organism. A low-copy quantity plasmid could be higher to attenuate poisonous results and keep away from killing your bacterial cultures.
  • Mutant Research
    You’ve got mutated your enzyme of curiosity. Now you need to evaluate its exercise to the wild-type enzyme in a physiological context (i.e., rework it into native host cells).

    Low-level expression from a single copy is often a greater choice to extend the possibilities of physiologically related measurements and to evaluate in vivo phenotypes.

    Over-expressed proteins could generate synthetic phenotypes, false protein-protein interactions, and structural points throughout the protein itself, resulting in complicated and unreliable outcomes. (Everyone knows that outcomes could be complicated sufficient with out additional complicating issues!)

How Can We Manipulate Plasmid Copy Quantity?

For the explanations given above, it may be advantageous to have a number of plasmids with completely different copy numbers to select from as you perform your analysis. A lot effort has gone into understanding how plasmid replication is managed, paving the way in which for us to govern this plasmid copy quantity. Let’s take a look at a few choices accessible to us.

Induced Amplification by Temperature Shift and Altering Bacterial Development Fee

You’ll be able to improve the copy quantity for some plasmids by rising the host at elevated temperatures. This could possibly be the case for pBR22 as a result of the bacterial development price influences the fine-tuning of the RNA I/RNA II regulation. [1] 

Chloramphenicol Amplification

Chloramphenicol amplification works for a lot of low-copy plasmids containing the pMB1 origin [2]. The primary steps are:

  • Publicity of the host bacterial tradition to the antibiotic chloramphenicol inhibits bacterial protein synthesis.
  • This results in chromosomal replication inhibition as a result of this additionally depends on ongoing protein synthesis and inhibition of cell division.
  • Plasmids solely require extra long-lived proteins and proceed to duplicate despite the fact that chromosomal replication and cell division have stopped.
  • Ultimately, plasmid replication stops when the cell turns into exhausted (proteins used up), however the common copy quantity can have elevated considerably.

Unintentional Manipulation of Plasmid Copy Quantity

As talked about above, the plasmid insert also can affect copy quantity. For instance, a high-copy pUC plasmid could replicate at medium or low copy numbers when ligated to very giant DNA inserts, leading to decrease plasmid yields than anticipated.

This decreased yield is as a result of plasmid replication is a metabolic burden for the host cell. If the burden turns into too giant (e.g., large inserts, elevated development temperature), plasmid-bearing cells will grow to be much less environment friendly, and development will decelerate.

Ultimately, the tradition will grow to be dominated by present plasmid-free cells, resulting in low plasmid yield.

Plasmid Copy Quantity Summarized

Plasmid copy quantity can affect experiments, and whereas it’s interesting to have high-copy numbers in your plasmids, there are some purposes the place low plasmid copy numbers are finest. You’ll be able to obtain the specified copy quantity through the use of an acceptable plasmid or copy quantity manipulation.

Have you learnt of some other strategies to extend the plasmid copy quantity? In case you have some concepts not talked about right here, we’d love to listen to from you!

Don’t overlook to take a look at the associated articles on the origins of replication and the high-copy pUC18 plasmid.

This text was initially printed on January 14, 2015. Reviewed and up to date Could, 2022.


  1. Lin-Chao S, Bremer H. 1986. Impact of the bacterial development price on replication management of plasmid pBR322 in Escherichia coli. Mol Gen Genet. 203(1):143–9.
  2. Howe C. Vectors, transformations and hosts. In Gene Cloning 2nd Ed. Cambridge College Press, 2007. ISBN: 9780521521055

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