When should you use a stringent plasmid?

Question

I was wondering if anyone had good examples of when you would want to use a stringent plasmid vs. a relaxed plasmid in a research setting.

Answer 1

The three general advantages are:

• Lower expression. If your plasmid is expressing something toxic to cells at high levels, reducing the amount of DNA can reduce expression.
• Lower plasmid burden. If you are concerned about the stress imposed by maintaining many copies of the plasmid, low-copy plasmids are helpful.
• Population homogeneity. Relaxed plasmids tend to replicate such that there will be a wide distribution of # plasmid copies/cell. Meanwhile, stringent plasmid replication is tightly coupled to the cell cycle itself, so the number varies less. In certain cases it may be desirable to control the number of plasmids per cell.