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

Department of Cell Biology, University of Auckland, Auckland, New Zealand
Department of Microbiology, Yale University School of Medicine, New Haven, Connecticut 06510

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.