Abnormal Excision and Transfer of Chromosomal Segments by a Strain of Escherichia coli K-12

PB15 is an Hfr strain ofEscherichia coli K-12. It arose from an F′ strain carrying a temperature-sensitive F-gal by an event which blocked the detachment of F-gal in the normally reversible integration process. In PB15, the detachment of F-gal by a second mechanism can now be detected: this mechanism results in the excision and transfer of extended chromosomal segments which include the integrated F-gal ; the excised segments are inferred to have circularized. Their excision, which is independent of therecA + allele, occurs at an unusually high rate during conjugation; a mutant F-initiator protein is suggested as the cause of this phenomenon. After their establishment in recipients, the enlarged F-genotes undergo further deletions of included donor genes by a process which is againrecA + -independent. In Rec+ , but not in Rec− , cells, a high proportion of the deleted fragments are rescued by integration into the recipient's chromosome.