Inhibition of influenza viral mRNA synthesis in cells expressing the interferon-induced Mx gene product

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Abstract
Interferons alpha and beta induce an efficient antiviral state against influenza virus in mouse cells that possess the Mx gene, but not in mouse cells that lack this gene. In Mx-containing cells treated with interferon the amount of viral mRNA synthesized as a result of primary transcription is drastically reduced. Only two viral mRNAs could be detected by Northern analysis and by translating the poly(A)+ RNA from infected cells in wheat germ extracts: a reduced amount of the mRNA for nonstructural protein 1 and an even lower amount of the mRNA for the matrix protein. The other viral mRNAs were not made in detectable amounts. In addition, the rate of viral mRNA synthesis catalyzed by the inoculum transcriptase, measured by in vitro RNA synthesis catalyzed by permeabilized cells, was severely inhibited. In contrast, interferon treatment of cells lacking the Mx gene had little or no effect on either the steady-state level or the rate of synthesis of viral mRNAs made by the inoculum transcriptase. These results indicate that the interferon-induced Mx gene product, a 75,000-molecular-weight protein that accumulates in the nucleus, inhibits influenza viral mRNA synthesis which occurs in the nucleus. No Mx-specific effect acting directly on viral protein synthesis in the cytoplasm was observed.