USE OF A TRANSPOSON (TNDIF) TO OBTAIN SUPPRESSING AND NONSUPPRESSING INSERTIONS OF THE DIF RESOLVASE SITE OF ESCHERICHIA-COLI

The dif locus is a RecA-independent recombination site, located in the terminus region of the chromosome of Escherichia coIi. This site func tions to reduce circular dimer chromosomes to monomers before cell div ision. Strains lacking this site exhibit the Dif phenotype, in which a fraction of the cells form extended filaments with abnormal nucleoids , and the SOS system is induced. We have used a transposon (Tndif), as well as linear transformation, to position dif in 19 locations around the chromosome. All of the suppressing insertions that we obtained we re within 10 kb of the normal site, even in strains in which the norma l symmetry between the origin of replication and dif had been altered by 200 kb. We also observed that the nonsuppressing insertions in the terminus region became suppressing if a deletion occurred that extende d from the ectopic site up to or past the normal location of dif. We p ropose that dif is normally located at the center of converging polari ties in the terminus region and that deletions that restore suppressio n do so by placing ectopic sites once again at the center of this pola rity. Similar results and conclusions are described in this issue.