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

Abstract

PB15 is an Hfr strain of Escherichia 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 the recA + 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 again recA + -independent. In Rec + , but not in Rec − , cells, a high proportion of the deleted fragments are rescued by integration into the recipient's chromosome.