RESTRICTION OF THE ACTIVITY RECOMBINATION SITE DIF TO A SMALL ZONE OFTHE ESCHERICHIA-COLI CHROMOSOME

The recombination site dif is the target on the Escherichia coli chrom osome 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 con trol over dif activity, also analyzed in this issue, is demonstrated h ere by translocation of dif to a series of loci close to the normal lo cus. We found that the site is physiologically active only within a na rrow zone around its natural position. Competence for dif activity doe s not depend on the sequence of the normal dif activity zone (DAZ), be cause Delta(dif) deletions larger than the DAZ result in Dif(+) bacter ia when dif is reinserted at the junction point. Although dif maps whe re 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(-) mu tation allows replication to terminate far away from it. Preliminary d ata suggest the possibility that specialized sequences separate the co mpetent zone from the rest of the chromosome. We suspect that these se quences are members of a set of sequences involved in a polarized proc ess of postreplicative reconstruction of the nucleoid structure. We pr opose that this reconstruction forces catenation links between sister chromosomes to accumulate within the DAZ, where they eventually favor recombination at dif.