Increased amplification of plasmids pBR322 and pBR327 by low concentrations of chloramphenicol.

The replication and amplification of plasmids pBR322 and pBR327 is maximal during partial inhibition of protein synthesis by low concentrations (10-20 micrograms/ml) of chloramphenicol in rich medium (LB). In this manner, 5- to 10-fold greater yields of plasmid DNA at 2- to 5-fold greater initial purity (less protein and chromosomal DNA content) can be obtained than by using the standard high concentrations of chloramphenicol. These results have been obtained with an improved method of plasmid quantitation in unfractionated bacterial lysates.