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Plasmid types

This is the division relevant to the genetic engineering and transforming target cells. There is also another division of plasmids according to their function (like resistance plasmids, degradative plasmids, virulence plasmids, etc.), but this is not really important for the subject, so I won’t cover it here.

Low-copy number plasmids

The main feature of these plasmids is that they replicate in low numbers in host cell. This enables the metabolism of a host cell to stay at low level, enabling the cell to reproduce moderately. They are easily constructible and they can be used to study genes in basic metabolic pathways, or to clone genes whose overexpression can kill the cell. They are also known as stringent plasmids, as their replication is “bounded” to, or limited by the replication of the host cell DNA.

Subtype of low-copy plasmids are runaway plasmids. They are sensitive to temperature and their expression (replication) remains low in the optimum temperature. If the temperature increases too much, they get overexpressed, which results in the death of the cell.

High-copy number plasmids

Opposite from low-copy number plasmids are high-copy number plasmids. The name tells for itself – they are able to replicate in high numbers after they enter into the host cell. They are also known as relaxed plasmids, because they are not limited by the replication of the host cell DNA. They can be used for the production of a certain protein which is needed in high ammount.

Conjugative plasmids

These plasmids contain mob (mobilizing) and tra (transfer) genes which help them transfer to the bacteria on their own. Tra genes are responsible for the formation of F-pilus – a bridge between two cells through which the plasmid can transfer itself. They usually have high molecular weight and they replicate only into few copies (1 to 3 per cell). Example of conjugative plasmids is F-plasmid.

Non-conjugative plasmids

These plasmids are the opposite of conjugative plasmids. They do not contain mob or tra genes and are therefore incapable of initiating the process of conjugation, so they need help from conjugative plasmids in order to be transferred. They have low molecular weight and they usually exist in multiple copies per cell.

Mobilizable plasmids

These plasmids are somewhere in between of conjugative and non-conjugative ones, because they contain mob genes, so they can use conjugative plasmids in order to transfer to other cells.