Excisionase

Excisionase, commonly referred to as Xis, is a small phage-encoded protein that acts as a recombination directionality factor to promote the excision of prophage DNA from the host chromosome, most notably in bacteriophage lambda.

During excision, Xis works with the phage-encoded integrase (Int) at the attachment sites attL and attR to catalyze recombination that yields attP and attB, effectively circularizing the prophage genome and restoring host DNA. Xis binds DNA specifically and promotes structural changes in the recombination complex, biasing the reaction toward excision rather than integration.

The protein is typically small and is expressed in a tightly regulated manner during the switch from lysogeny to the lytic cycle. Without Xis, excision occurs at low frequency, and efficient transition to the lytic state can be impeded.

Xis is encoded by the prophage and is not part of the host genome; homologous excision directionality factors exist in other phages that utilize comparable excision mechanisms. Its function helps determine the outcome of phage-host recombination events and is central to the lifecycle decision between lysogeny and the lytic program.