Inversion of the 4.2‐kb C segment flanked by 0.6‐kb inverted repeats on the bacteriophage P1 genome is mediated by the P1‐encoded site‐specific cin recombinase. The cin gene lies adjacent to the C segment and the C inversion cross‐over sites cixL and cixR are at the external ends of the inverted repeats. We have sequenced the DNA containing the cin gene and these cix sites. The cin structural gene consists of 561 nucleotides and terminates at the inverted repeat end where the cixL site is located. Only two nucleotides in the cixL region differ from those in the cixR and they are within the cin TAA stop codon. The cin promoter was localized by transposon mutagenesis within a 0.1‐kb segment, which contains probable promoter sequences overlapping with a ‘pseudo‐cix’ sequence cixPp. In a particular mutant, integration of an IS1‐flanked transposon into the cin control region promoted weak expression of the cin gene. The cin and cix sequences show homology with corresponding, functionally related sequences for H inversion in Salmonella and with cross‐over sites for G inversion in phage Mu. Based on a comparison of the DNA sequences and of the gene organizations, a possible evolutionary relationship between these three inversion systems and the possible significance of the cixPp sequence in the cin promoter are discussed.

Plasmid pl5B of Escherichia coli 15T-carries a 3.5-kilobase segment that undergoes frequent DNA inversion mediated by the DNA inversion enzyme Min, a member of the Din family of site-specific recombinases. While the previously described Din inversion systems invert a DNA segment between two crossover sites in inverted orientation, the Min system produces more complex DNA rearrangements. These have been physically characterized by electron microscopy and by restriction cleavage analysis. The results can best be explained by a model that involves six crossover sites (called mix) and predicts 240 isomeric forms of the invertible region.The model was confirmed by sequencing the six mix sites in plasmids that contain the invertible DNA segments in a frozen configuration. All mix sites fit the dix consensus sequence, and they are all good substrates for DNA inversion when carried in inverted orientation. Recombination between two mix sites in direct orientation was rare, in line with the notion that Din inversion systems are topologically biased to the inversion reaction. Another recently described multiple inversion system, the shufflon of the E. coli plasmid R64, is neither functionally nor structurally related to the Min system of pl5B.gene also encodes a recombinational enhancer element (unpublished data). In contrast to all previously described Din systems, which invert a DNA segment between two crossover sites, the Min system (Min for multiple DNA inversion) causes more complex DNA rearrangements. The characterization of this mobile DNA region of p15B is the subject of this report.During our studies another example of clustered DNA inversion segments has been described on the R64 plasmid of E. coli (10). In this system, which has been named "shufflon," four DNA segments can invert independently or in groups resulting in complex DNA rearrangements. DNA inversion is brought about by site-specific recombination mediated by the Rci recombinase. Although the Min system of pl5B and the shufflon of R64 show a similar clustering of overlapping invertible DNA segments, they are not related because the DNA sequences of neither the recombinase genes (unpublished data) nor, as we show here, the crossover sites are homologous. In turn, the Rci enzyme shares significant sequence homologies with the recombinases of the integrase family (11).The prokaryotic site-specific DNA recombination systems that have been well characterized so far can be classified into two groups on the basis of their structural and functional homologies and of their mechanistic properties (1,2). One group of loosely related enzymes includes the integrases of phage A (3) and other lambdoid phages, the Cre recombinase of phage P1 (4), and the eukaryotic FLP enzyme of yeast (5).The other group (6) is subdivided into two families: one comprises the resolvases of the Tn3 family of transposons, while the other family consists of a number of DNA invertases (Din family). Site-specific recombinases for the latter are found in the genomes of phages P1 (Ci...

No abstract

Bacteriophage P1 mutants with the 8.86-kb region between the invertible C-segment and the residential IS1 element deleted from their genome are still able to grow vegetatively and to lysogenize stably, but they show several phenotypic changes. These include the formation of minute plaques due to delayed cell lysis, the abundant production of small-headed particles, a lack of specific internal head proteins, sensitivity to type I host restriction systems, and altered properties to mediate generalized transduction. In the wild-type P1 genome, the accessory genes encoding the functions responsible for these characters are localized in the darA operon that is transcribed late during phage production. We determined the relevant DNA sequence that is located between the C-segment and the IS1 element and contains the cin gene for C-inversion and the accessory genes in the darA operon. The darA operon carries eight open reading frames that could encode polypeptides containing >100 amino acids. Genetic studies indicate that some of these open reading frames, in particular those residing in the 5' part of the darA operon, are responsible for the phenotypic traits identified. The study may contribute to a better comprehension of phage morphogenesis, of the mobilization of host DNA into phage particles mediating generalized transduction, of the defense against type I restriction systems, and of the control of host lysis.

Plasmid p15B is a bacteriophage P1-related resident of Escherichia coli 15T-. Both genomes contain a segment in which DNA inversion occurs, although this part of their genomes is not identical. This DNA segment of p15B was cloned in a multicopy vector plasmid. Like its parent, the resulting plasmid, pAW800, undergoes complex multiple DNA inversions: this DNA inversion system is therefore called Min. The min gene, which codes for the p15B Min DNA invertase, can complement the P1 cin recombinase gene. The Min inversion system is thus a new member of the Din family of site-specific recombinases to which Cin belongs. The DNA sequence of the min gene revealed that Min is most closely related to the Pin recombinase of the e14 defective viral element on the E. coli K12 chromosome. Like other members of the Din family, the min gene contains a recombinational enhancer element which stimulates site-specific DNA inversion 300-fold.