A system for study of coronavirus mRNA synthesis: a regulated, expressed subgenomic defective interfering RNA results from intergenic site insertion.

A system that exploits defective interfering (DI) RNAs of mouse hepatitis virus (MHV) for deciphering the mechanisms of coronavirus mRNA transcription was developed. A complete cDNA clone of MHV DI RNA containing an inserted intergenic region, derived from the area of genomic RNA between genes 6 and 7, was constructed. After transfection of the in vitro-synthesized DI RNA into MHV-infected cells, replication of genomic DI RNA as well as transcription of the subgenomic DI RNA was observed. S1 nuclease protection experiments, sequence analysis, and Northern (RNA) blotting analysis revealed that the subgenomic DI RNA contained the leader sequence at its 5' end and that the body of the subgenomic DI RNA started from the inserted intergenic sequence. Two subgenomic DI RNAs were synthesized after inserting two intergenic sites into the MHV DI RNA. Metabolic labeling of virus-specific protein in DI RNA replicating cells demonstrated that a protein was translated from the subgenomic DI RNA, which can therefore be considered a functional mRNA. Transfection study of gel-purified genomic DI RNA and subgenomic DI RNA revealed that the introduction of the genomic DI RNA, but not subgenomic DI RNA, into MHV-infected cells was required for synthesis of the subgenomic DI RNA. A series of deletion mutations in the intergenic site demonstrated that the sequence flanking the consensus sequence of UCUAAAC affected the efficiency of subgenomic DI RNA transcription and that the consensus sequence was necessary but not sufficient for the synthesis of the subgenomic DI RNA.