Purified defective-interfering (DI) particles of Semliki Forest virus are unable to carry out any of the steps in virus multiplication except uncoating. Cells co-infected with DI particles and standard virus contain several virus-specified RNA species (DI particle-specific species) absent from cells infected with standard virus alone. Moreover, synthesis of the virus-specified components distinctive of standard virus-infected cells is reduced. The DI particle-specific RNA species comprise two poly A-containing single-stranded RNAs (DIss1 and DIss2), identical to those found in purified DI particles, two double-stranded RNAs (RFs) and a new size class of replicative intermediate (RI).

Hybridization experiments showed that the nucleotide sequences of DIss1 and DIss2 (i) are present in the 42S genome of standard virus but absent from the 26S RNA — the RNA from standard virus-infected cells which encodes the structural proteins of the virion (Clegg & Kennedy, 1975) and (ii) are complementary to the negative strands of the DI particle-specific RFs and RI. Oligonucleotide fingerprinting revealed extensive nucleotide sequence homology between DIss1 and DIss2. Analysis of the mRNA complement of standard virus-infected, co-infected and uninfected cells strongly indicated that neither DIss1 nor DIss2 can serve as a functional messenger RNA.

From these studies we propose a mechanism for the multiplication of and interference by DI particles of Semliki Forest virus.


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