F'-coded, temperature-sensitive lambda cI857 repressor gene for easy construction and regulation of lambda promoter-dependent expression systems.

We describe the construction and properties of an F' factor which carries the temperature-sensitive cI857 allele of the repressor gene of coliphage lambda and which lacks the lambda cro function. This episome can easily be transferred to any F- and F' Escherichia coli strain, thus facilitating the construction and regulation of lambda promoter-dependent expression systems without the use of defective prophages.