On the Two Deoxyribonucleases, Kl and K2, Isolated from Mycelia of Aspergillus oryzae* IV. Isolalion of Thymidylic Acid Rich Fragments from DNA of Bacteriophage fi by Digestion with DNase K2

A single stranded DNA of bacteriophage fi was digested with DNase K2. Its diges-tion products with short chain lengths were found to be generally G-rich. Polynucleo-tides with a chain length of 30—50 nucleotides were recovered from a DEAE-cellulose column and purified by gel nitration. The polynucleotides contained about 50 molar percents of thymidylic acid (T-rich DNA fragments). The T-rich DNA fragments showed hyperchromism, and the heat denatured DNA was renatured on lowering of tempera-ture with a high degree of renaturability. The existence of a certain kind of secondary structure in phage ft DNA was discussed. Degradation of native and heat denatured forms of DNA by DNases Kl and K2 was reported previously (1—3). Lately, degrada-tion of single-stranded DNA by DNase K2 was investigated, paying special attention to the isolation and characterization of DNA fragments with comparatively long chain lengths. Expectation was that the long chained DNA fragments might be pyrimidine-rich (2, 3). Some properties of the thymidylic acid rich DNA fragments obtained from bacterio-phage fi DNA will be described. MATERIALS AND METHODS Phagi fi DNA—The DNA was prepared by the methods reported by MARVIN and SCHALLER (4). The preparation showed an EnofEzx value of 2.21 and an EJ»/EIIO value of 1.71. The SJO value in the standard saline-citrate solution was 21 (uncorrected), a value almost of the same order as that of intact 0x 174 DNA already reported (22.6 and 23.3, Eiger and Doty, 1965) (5). DigiUion of fi DNA with DNase K2—The ft DN