Replicons and Compatibility

The number of plasmids per cell is determined by the origin of replication, also known as the replicon. Relaxed plasmids produce many copies of themselves per host cell, while stringent plasmids produce only a few copies per cell. In addition to the number of plasmids per cell, we need to be mindful of the mechanism for plasmid replication. It is not possible to maintain in a single cell two different plasmids that use the same mechanism for replication. Therefore, plasmids fall into incompatibility groups. You cannot use two plasmids in a single cell if the plasmids are the the same incompatibility group.

It is worth noting the effect of chloramphenicol on plasmid copy number. If a plasmid uses RNA as its positive regulatory molecule, then it does not require protein synthesis to replicate which is not true for chromosomal replication. Therefore, it the protein synthesis inhibitor chloramphenicol is given to cells with these plasmids, the number of plasmids relative to the number of chromosomes goes way up (thousands of copies per cell). In fact, after a few hours, the plasmids can represent as much as half of all DNA in the cell. Plasmids that use the protein RepA as a positive regulator (e.g., pSC101) cannot be amplified by chloramphenicol. However, if your plasmid contains the chloramphenicol resistance gene (forming the homotrimer chloramphenicol acetyltransferase type I - BioBrick part number P1004 and PDB ID 1pd5), then the chloramphenicol is inactivated and plasmid copy number is not affected for any replicons.

Replicons and Copy Number

Incompatibility Groups

Both tabels from: Sambrook and Russell. 2001. Molecular Cloning: A laboratory manual. 3^rd Ed. Cold Spring Harbor Laboratory Press.