Site-specific recombination: Xis-independent excisive recombination of bacteriophage lambda

Abstract

The integration of bacteriophage lambda into the Escherichia coli chromosome depends on the phage-encoded Int protein; prophage excision depends on Int and a second phage function, Xis. Limited excisive recombination has been observed in vivo with certain xis mutants, suggesting that Int may be able to carry out excision without Xis. We report here that purified Int protein carries out lambda site-specific excisive recombination in vitro in the absence of Xis. This reaction requires host factors derived from a non-lysogenic E. coli strain and is influenced strongly by ionic strength. Excision in the absence of Xis occurs slowly at low salt concentrations (40 m m-NaCl) and very little excision occurs at high salt concentrations (100 m m-NaCl). In the presence of Xis, excisive recombination proceeds rapidly at both low and high ionic strengths. These observations are consistent with previous experiments that suggested the partial dispensability of Xis for excision.