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, th e target site of the XerC/XerD-dependent recombination system in Esche richia coli. Using a technique which allows very sensitive detection o f plasmid loss, we show that recombination at this site, termed psi fo r 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 derivativ e of plasmid p15A and inserted into the chromosome in place of dif. In the first situation, psi activity requires accessory sequences and re sults in multimer resolution; in the second situation, it suppresses t he effects of the dif deletion and can promote intermolecular exchange s. Thus, psi is a site whose recombinational activity depends on the c ontext, the first in the cer/dif family known to exhibit such flexibil ity.