We have shown that a single defective-interfering (DI) particle of early (5th) passage Sindbis virus induces maximal amounts of interferon in an ‘aged’ primary chick embryo cell. The capacity of such DI particles to induce interferon is inactivated by small amounts of u.v. radiation (1/e dose = 232 ergs/mm). The 1/e dose for inactivation of the interferon-inducing capacity of infectious virus particles is 399 ergs/mm and for infectivity is 1010 ergs/mm. Pre-treatment with interferon blocks formation of interferon in response to either DI or infectious virus particles. Our results suggest that Sindbis virus genes must be expressed to form the interferon inducer, which is presumably a molecule of double-stranded (ds)RNA. We postulate that for interferon induction, the genomic RNA which codes for genes G and A must be translated into products whose concerted action produces a dsRNA molecule upon synthesis of a segment of RNA complementary to the genome. The RNA from early passage DI particles is sufficiently large (25S, 1.6 × 10 mol. wt.) to accommodate these genes, whereas the RNA from the late passage DI particles (20S, 1.0 × 10 mol. wt.) is not. Late (15th) passage DI particles do not induce interferon formation.


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