Anacystis nidulans infected by cyanophage AS-1 produces a site-specific endonuclease cleaving double-stranded DNA, as judged from the gel electrophoretic analysis of the reaction products and the determination of the terminal nucleotide at the 5′-end of the fragments. The enzyme was purified to near electrophoretic homogeneity. It has a molecular weight of 40,000 ± 4,000. Only Mg2+ ions are required for enzyme activity. Limit digestion was not obtained even after extensive digestion of substrate DNA with large amounts of the purified enzyme. This suggests that the endonuclease splits the substrate at more than one nucleotide sequence with a different efficiency for each sequence. The following observations suggest that the endonuclease takes part in the breakdown of host DNA in the infected cell: purified host DNA is degraded by, while cyanophage AS-1 DNA is protected against, the enzyme, and the appearance of the endonuclease during the lytic cycle coincides with the onset of intensive degradation of host DNA.
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