Although infective Newcastle disease virus (NDV) did not produce interferon in chick cells, brief heat treatment converted it to an inducer. Experiments in which mixtures of heated and unheated virus were used to induce interferon showed that a substance was produced in infected cells that inhibited interferon formation. Heat treatment of NDV caused the same rate of loss of infectivity and of virus particle RNA polymerase activity, and it was concluded that polymerase activity was essential for virus infectivity. It was also shown that some polymerase activity still existed in heat-inactivated virus able to induce interferon. Appropriate inactivation of the virus by u.v. irradiation or by treatment with β-propiolactone or at pH 2.5, also made the virus into an inducer of interferon, and in each case some virus polymerase activity was present in the inactivated particles. Both u.v. irradiation and β-propiolactone inactivated infectivity more rapidly than polymerase activity, while pH 2.5 treatment had the reverse effect.


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