Plasmid pSC101 harbors a recombination site, psi, which is able to resolve plasmid multimers and to substitute for the analogous chromosomal Escherichia coli site dif.

Plasmid pSC101 harbors a 28-bp sequence which is homologous to dif, the target site of the XerC/XerD-dependent 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 p15A 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 cer/dif family known to exhibit such flexibility.