Plasmid pSC101 harbors a 28-bp sequence which is homologous to dif, the target site of the XerC/XerDdependent recombination system in Escherichia coli. Using a technique which allows very sensitive detection of plasmid loss, we show that recombination at this site, termed psi for pSC101 stabilized inheritance, causes a moderate increase in pSC101 stability. The role of the psi sequence in site-specific recombination has been explored in two other contexts. It was cloned in a derivative of plasmid pl5A and inserted into the chromosome in place of dif. In the first situation, psi activity requires accessory sequences and results in multimer resolution; in the second situation, it suppresses the effects of the dif deletion and can promote intermolecular exchanges. Thus, psi is a site whose recombinational activity depends on the context, the first in the ceridif famil known to exhibit such flexibility.

The recombination site dif is the target on the Escherichia coli chromosome of the site-specific recombinases XerC and XerD. The dif/XerC-D system plays a role during the cell cycle, probably by favoring sister chromosome monomerization or separation. A phenomenon of regional control over dif activity, also analyzed in this issue, is demonstrated here by translocation of dif to a series of loci close to the normal locus. We found that the site is physiologically active only within a narrow zone around its natural position.Competence for dif activity does not depend on the sequence of the normal dif activity zone (DAZ), because ~(diD deletions larger than the DAZ result in Dif + bacteria when dif is reinserted at the junction point. Although dif maps where replication normally terminates, termination of replication is not the elicitor. A strain with a large inversion that places dif and its surrounding region close to oriC remains Dif*, even when a Tus-mutation allows replication to terminate far away from it. Preliminary data suggest the possibility that specialized sequences separate the competent zone from the rest of the chromosome. We suspect that these sequences are members of a set of sequences involved in a polarized process of postreplicative reconstruction of the nucleoid structure. We propose that this reconstruction forces catenation links between sister chromosomes to accumulate within the DAZ, where they eventually favor recombination at d/f.[Key Words." Site-specific recombination; dif site domain of activity; E. coli chromosome organization] Received January 10, 1996; revised version accepted March 20, 1996.The

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The frequency of excisive homologous recombination has been measured at various positions along the Escherichia coli chromosome. The reporter system makes use of a X c1857 prophage integrated by homologo4s recombination within TnS or TnlO transposons already installed at known positions in the E. coli chromosome. The excision frequency per cell and per generation was determined by monitoring the evolution of the relative number of temperature-resistant (cured) bacteria as a function of the age of the cultures. Excisions, due to RecA-dependent homologous exchanges, appeared to occur more frequently in the preferential termination zone for chromosome replication. The highest frequency of excision observed is compatible with a recombination event at each replication cycle in this region. On the basis of these data, we propose a model involving homologous recombination in the final steps of bacterial chromosome replication and separation.

The terminus region of the Escherichia coli chromosome is the scene of frequent homologous recombination. This can be demonstrated by formation of deletions between directly repeated sequences which flank a genetic marker whose loss can be easily detected. We report here that terminal recombination events are restricted to a relatively large terminal recombination zone (TRZ). On one side of the TRZ, the transition from the region with a high excision rate to the normal (low) excision rates of the rest of the chromosome occurs along a DNA stretch of less than 1 min. No specific border of this domain has been defined. To identify factors inducing terminal recombination, we examined its relation to two other phenomena affecting the same region, site-specific recombination at the dif locus and site-specific replication pausing. Both the location and the efficiency of terminal recombination remained unchanged after inactivation of the dif-specific recombination system. Similarly, inactivation of site-specific replication pausing or displacement of the replication fork trap so that termination occurs about 200 kb away from the normal region had no clear effect on this phenomenon.Therefore, terminal recombination is not a direct consequence of either dif-specific recombination or replication termination. Furthermore, deletions encompassing the wild-type TRZ do not eliminate hyperrecombination. Terminal recombination therefore cannot be attributed to the activity of some unique sequence of the region. A possible explanation of terminal hyperrecombination involves nucleoid organization and its remodeling after replication: we propose that postreplicative reconstruction of the nucleoid organization results in a displacement of the catenation links between sister chromosomes to the last chromosomal domain to be rebuilt. Unrelated to replication termination, this process would facilitate interactions between the catenated molecules and would make the domain highly susceptible to recombination between sister chromosomes.The region of the Escherichia coli chromosome in which replication terminates is implicated in two recombination events occurring at high frequencies in cells with a wild-type cell cycle machinery. One is site-specific recombination, controlled by the XerCD recombinases (4), which takes place at a unique site, dif, located at 34 min on the genetic map (3,5,20).The dif-specific recombination probably participates in chromosome dimer resolution, since bacteria deleted for the dif locus, or made XerC-, have a tendency to form filaments (3,20), but the exact physiological role of dif-mediated recombination is not precisely known. The other very frequent event is homologous recombination between directly repeated sequences (24). The position where the phenomenon was the most striking (the zdd263::TnS locus) maps between dif and the first replication terminator acting on forks moving rightward in Fig. 1B, terC pothesis concerning the mechanism and the role for the hyperrecombination in the terminus region has been...