High mol. wt. DNA was extracted from lambda lysogens and was shown to be infectious. Its infectivity was due to prophage DNA integrated into the host chromosome rather than to DNA released from mature phage particles, as established by the following criteria: the titre of infectious DNA exceeded by 100-fold the titre of infectious units present before DNA extraction; mild shear selectively reduced prophage DNA infectivity to 2% of the unsheared DNA while lambda phage DNA infectivity retained 50% of its infectivity; DNA extracted from an (lambda c857 ts) lysogen yielded 200 times as many plaques on than on spheroplasts. Thus lambda prophage DNA infectivity depends on expression of the excision gene while the infectivity of non-integrated forms of lambda does not. About 10 genome equivalents of DNA yielded one infectious centre unit in this assay system; this high infectivity should make prophage DNA a useful marker in genetic transformation experiments.


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