THE DIF RESOLVASE LOCUS OF THE ESCHERICHIA-COLI CHROMOSOME CAN BE REPLACED BY A 33-BP SEQUENCE, BUT FUNCTION DEPENDS ON LOCATION

The dif locus (deletion-induced filamentation) of Escherichia coli is a resolvase site, located in the terminus region of the chromosome, th at reduces chromosome multimers to monomers. In strains in which this site has been deleted, a fraction of the cells is filamentous, has abn ormal nucleoid structure, and exhibits elevated levels of the SOS repa ir system. We have demonstrated that a 33-bp sequence, which is suffic ient for RecA-independent recombination and which shows similarity to the cer site of pColE1, suppresses the Dif phenotype when inserted in the terminus region. Flanking sequences were not required, since suppr ession occurred in strains in which dif as well as 12 kb or 173 kb of DNA had been deleted. However, location was important, and insertions at a site 118 kb away from the normal site did not suppress the Dif ph enotype. These sites mere otherwise still functional, and they exhibit ed wild-type levels of RecA-independent recombination with dif-contain ing plasmids and recombined with other chromosomal dif sites to cause deletions and inversions. It is proposed that the functions expressed by a dif site depend on chromosome location and structure, and analysi s of these functions provides a way to examine the structure of the te rminus region.