An endonuclease, which was originally identified for its RNA polymerase inhibitory activity, was isolated from rat liver endoplasmic reticulum. The enzyme yields on gel chromatography four active fractions of different molecular weights (Mr 5.3 X 10(4), 9 X 10(4), 1.55 X 10(5) and Sephacryl S-200 fraction at V0). Each fraction contains polypeptide chains which give a single band on sodium dodecylsulphate electrophoresis (Mr 5.4 X 10(4). This indicates that the enzyme is an oligomeric protein and each of its subunits exhibits the same or very similar molecular weights. Deoxyribonucleoside and ribonucleoside triphosphates can bind to the endoplasmic reticulum nuclease. Binding is enhanced in the presence of divalent cations particularly Mg2+. The enzyme exhibits mainly RNase activity but can also degrade denatured DNA and DNA . RNA hybrids which contain breaks in one of the two strands. Poly(A) and mainly poly(U) are most susceptible to its nucleolytic activity whereas poly(C) is completely resistant.
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