Purification and properties of ribonuclease H of calf thymus.

Ribonuclease H of calf thymus has been purified better than 3000-fold to yield an almost homogeneous preparation. The enzyme, which comprises about 0.03% of the total protein in the initial extract, is a slightly acidic protein (pI = 4.95) of molecular weight of about 64,000, possibly composed of subunits. The enzyme requires a metal ion for activation; the conditions for activation by Mg, Co, and Mn are described. It is inhibited by S-adenosylmethionine. The substrates cleaved were poly(dT).poly(rA) and the DNA-RNA hybrid made from phage f1 DNA; ribosomal RNA was not attacked.