HYPERRECOMBINATION IN THE TERMINUS REGION OF THE ESCHERICHIA-COLI CHROMOSOME - POSSIBLE RELATION TO NUCLEOID ORGANIZATION

The terminus region of the Escherichia coli chromosome is the scene of frequent homologous recombination. This can be demonstrated by format ion of deletions between directly repeated sequences which flank a gen etic marker whose loss can be easily detected. We report here that ter minal recombination events are restricted to a relatively large termin al recombination zone (TRZ). On one side of the TRZ, the transition fr om the region with a high excision rate to the normal (low) excision r ates 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 ide ntify factors inducing terminal recombination, we examined its relatio n to two other phenomena affecting the same region, site-specific reco mbination at the dif locus and site-specific replication pausing. Both the location and the efficiency of terminal recombination remained un changed after inactivation of the dif-specific recombination system. S imilarly, inactivation of site-specific replication pausing or displac ement of the replication fork trap so that termination occurs about 20 0 kb away from the normal region had no clear effect on this phenomeno n. Therefore, terminal recombination is not a direct consequence of ei ther dif-specific recombination or replication termination. Furthermor e, deletions encompassing the wild-type TRZ do not eliminate hyperreco mbination. Terminal recombination therefore cannot be attributed to th e activity of some unique sequence of the region. A possible explanati on of terminal hyperrecombination involves nucleoid organization and i ts remodeling after replication: we propose that postreplicative recon struction of the nucleoid organization results in a displacement of th e catenation links between sister chromosomes to the last chromosomal domain to be rebuilt. Unrelated to replication termination, this proce ss would facilitate interactions between the catenated molecules and w ould make the domain highly susceptible to recombination between siste r chromosomes.