Transcription strategy of coronaviruses: fusion of non-contiguous sequences during mRNA synthesis.

MHV replicates in the cell cytoplasm and viral genetic information is expressed in infected cells as one genomic sized RNA ( mRNA1 ) and six subgenomic mRNAs. The seven RNAs were assumed to have common 3' ends of the size of RNA7 , the smallest RNA. The data reported here, show that this model is too simple and that the mRNAs are composed of a leader and body sequence. Electron microscopic analysis of hybrids formed between single stranded cDNA copied from mRNA7 and genomic RNA or mRNA6 shows that genomic RNA, mRNA6 and mRNA7 have common 5' terminal sequences. Furthermore, nucleotide sequence analysis shows that the nucleotide sequence of the 5' end of mRNA7 diverges from the corresponding region of the genome just upstream from the initiation codon of the nucleocapsid gene. Because the synthesis of each mRNA is inactivated by UV irradiation in proportion to its own length, the subgenomic mRNAs are apparently not produced by the processing of larger RNAs. The available data have to be explained by translocation of the polymerase/leader complex to specific internal positions on the negative strand. In this way the leader and body sequences are joined together by a mechanism completely different from conventional RNA splicing but nevertheless giving the same end result.