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#Xis protein

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Recombination

Xis protein primarily promotes

Directionality of lambda recombination controlled by accessory proteins dictating which attachment sites synapse. Integration intasome formed by Int plus IHF favors attP x attB recombination generating attL and attR flanking prophage. For prophage escape after induction, Xis protein synthesized binds X binding sites in attR introducing bends that prevent Int from occupying P-type sites while favoring excisive contacts. Xis also contacts Fis and Int N-terminal domain redirecting tetramer towards attL x attR excisive mode. Thus Xis shifts equilibrium from integrative to excisive state promoting

Ref: Watson et al., Molecular Biology of the Gene, 7th ed., Chapter 12: Xis Promotion of Lambda Excision

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Recombination

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

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