Characterization of the integration protein of bacteriophage lambda as a site-specific DNA-binding protein.

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Abstract
The Int protein specified by bacteriophage .lambda. is required for the recombination event that integrates the viral DNA into the host [Escherichia coli] genome at its specific attachment site. Using a DNA-binding assay, the Int protein was purified, and some of the features of its binding specificity and regulation were studied. The DNA-binding activity is attributed to Int protein because the activity is eliminated by a nonsense mutation or a deletion in the int gene, and is rendered thermolabile by temperature sensitive mutations in the int gene. The DNA-binding activity is specific for DNA carrying an appropriate attachment site, suggesting that Int protein directs the sequence-specific recognition essential for integrative recombination. The specific DNA-binding activity is also missing after infection by phage carrying mutations in the cII and cIII regulatory genes of .lambda.. This confirms that regulation of the int and cI genes by cII/cIII provides for coordinate regulation of both major events of the lysogenic response, establishment of repression and insertion of viral DNA.