The invertible P-DNA segment in the chromosome of Escherichia coli

In the chromosome of many strains of Escherichia coli K12 the excisable element e14 is found, which contains an invertible DNA region. This invertible P region, and the gene responsible for the inversion (pin) were cloned, together with other e14 sequences. The element e14 contains a gene which kills the host cell. This can be repressed by a function also coded by e14. The kil and repressor genes as well as the attachment site of the element were mapped in different regions of the element. The invertible segment and pin gene were sequenced. The invertible segment is 1794 bp long, and contains one large internal open reading frame of 879 bp and reading frames which overlap the end pont of the invertible segment. Although pin highly homologous to gin of phage Mu, neither the genetic organization of the P segment nor the sequence of the putative proteins resemble the invertible G segment of phage Mu (which codes for genes involved in tail fiber assembly). The complete DNA sequences of both invertible segments were screened for homology. No resemblance was found. The P segment is flanked by inverted repeat sequences of 16 bp. Comparison of these with related inversion systems points out that the recombination site maps probably within a 2-bp region. This cross-over site is contained within a short palindromic sequence (AAACC AA GGTTT) which is more or less conserved in the recombination sites of all related DNA invertases.