The repeated regions of Semliki Forest virus defective-inferfering RNA interferes with the encapsidation process of the standard virus.

Two different defective interfering RNAs of Semliki Forest virus have been cloned and sequenced previously. These molecules have repeated sequence blocks between unique terminal regions. The late gene region of SV40 virus has been replaced with the repeating unit detected in both defective-inferfering (DI) RNAs, and by complementation with a tsA mutant of SV40 a mixed stock of recombinant and helper virus was obtained. Upon infection of monkey kidney cells the recombinant expressed the repeated part of the DI RNA (svDI301 RNA). Superinfection of these cells with standard Semliki Forest virus showed that (i) the synthesis of SFV genomic RNA is marginally if at all affected by the svDI301 RNA, (ii) the svDI301 RNA is not replicated by SFV-RNA-dependent RNA polymerase, and (iii) packaging efficiency of the standard SFV genome RNA into virions is clearly decreased in the presence of svDI301 RNA. These results suggest that the terminal regions of the DI RNA molecule are required for efficient replication while the central repeated elements are involved in encapsidation.