Plasmids contain within them replicons that monitor the copy number of the plasmid; that is, how many plasmids exist in the particular host at any given time.  This is important, because too low of a copy number may result in plasmid loss through subsequent generations; likewise, too high of a copy number taxes the host’s systems and may cause deleterious effects through the sheer metabolic burden.  Replicons can be high copy number (up to 100’s of copies) or low copy number (2-3).

It is important to realize that a higher copy number indicates that all of the plasmid genes are being expressed at a higher rate; this causes a significant metabolic burden.

Some replicons may be temperature sensitive; for instance, the pKN402 origin maintains about 80 plasmids per cell at 30 degrees, but this number rises to 500 plasmids per cell at 42 degrees. If we couple this with the pL system (lambda left promoter) and cI₈₅₇, the system will be of relatively low metabolic burden if we grow the culture at 30 degrees; then, when we want to produce product, we raise the temperature to 42 degrees, which has the simultaneous effect of increasing the copy number of the plasmids (to 500 per cell), and releasing cI₈₅₇ gene product from the pL promoters, allowing transcription to begin.

We can increase the gene copy number to increase expression, as well; that is, we simply place multiple copies of the gene on the vector, meaning there are now multiple points of initiation for transcription.