Xis protein is required for
Prophage excision is directionally opposite to integration and requires remodeling of recombination complex. Xis, encoded by defective phage gene, is small DNA-binding protein that binds cooperatively to three sites in attR, introducing additional bends and displacing IHF-dependent intasome structure. Xis binding favors attL and attR synapsis instead of attP and attB, stimulates excisive exchange, and inhibits reintegration pathway. Additionally Fis protein stabilizes excisive intasome. Coordinated expression of Xis and Int after SOS induction thus ensures precise excision of lambda genome res
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8: Xis and Excision of Lambda Prophage
