Insertion of a new transcriptional unit into the genome of mouse hepatitis virus

The subgenomic mRNAs of the coronavirus mouse hepatitis virus (MHV) are composed of a leader sequence, identical to the 5' 70 nucleotides of the genome, joined at distant downstream sites to a stretch of sequence that is identical to the 3' end of the genome. The points of fusion occur at intergenic sequences (IGSs), loci on the genome that contain a tract of sequence homologous to the 3' end of the leader RNA. We have constructed a mutant of MHV-A59 containing an extra IGS inserted into the genome immediately downstream of the 3'-most gene, that encoding the nucleocapsid (N) protein. We show that in cells infected with the mutant, there is synthesis of an additional leader-containing subgenomic RNA of the predicted size. Our study demonstrates that (i) an IGS can be a sufficient cis-acting element to dictate MHV transcription, (ii) the relative efficiency of an IGS must be influenced by factors other than the nucleotides immediately adjacent to the 5'AAUCUAAAC3' core consensus sequence or its position relative to the 3' end of the genome, (iii) a downstream IGS can exert a polar attenuating effect on upstream IGSs, and (iv) unknown factors prevent the insertion of large exogenous elements between the N gene and the 3' untranslated region of MHV. These results confirm and extend conclusions previously derived from the analysis of defective interfering RNAs.