It has been reported that T4 gene 49 controls a nuclease-like activity which attacks very fast sedimenting DNA (VFS-DNA) formed in cells infected with a mutant defective in gene 49 (Frankel, F. R., Batcheler, M. L., and Clark, C. K. (1971) J. Mol. Biol. 62: 439-463). The protein responsible for this activity was partially purified and shown to be an endonuclease. The DNase only attacks T4 DNA having a specific structure; besides VFS-DNA, it cleaves T4 DNA which contains gaps with an internal 5′-phosphorylated terminus. Native, denatured, and nicked T4 DNAs are not susceptible to the enzyme action. © 1978.