Transient suppression of F-plasmid incompatibility in a strain of Escherichia coli.

The incompatibility between F plasmids is transiently suppressed in Escherichia coli strain CSH54. As a result this strain is able to maintain two F' factors or an F' factor and a mini-F plasmid for considerably longer periods than normal strains. When selective pressure for two markers carried by two separate F's (or an F' and mini-F) is imposed on normal strains, the two plasmids tend to form a cointegrate structure which can be detected genetically by the joint transfer of both the markers upon mating. This does not happen in CSH54; instead, the two plasmids are maintained and transferred independently. Physical evidence for the maintenance of an F' and a mini-F plasmid is provided by agarose gel electrophoresis.